Accepted Manuscript Title: Development and Validation of an LC-MS/MS Based Method for Quantification of 25 Hydroxyvitamin D2 and 25 Hydroxyvitamin D3 in Human Serum and Plasma Author: Stanley Weihua Zhang Wenying Jian Sheryl Sullivan Banu Sankaran Richard W. Edom Naidong Weng David Sharkey PII: DOI: Reference:
S1570-0232(14)00306-7 http://dx.doi.org/doi:10.1016/j.jchromb.2014.05.006 CHROMB 18929
To appear in:
Journal of Chromatography B
Received date: Revised date: Accepted date:
28-2-2014 29-4-2014 4-5-2014
Please cite this article as: S.W. Zhang, W. Jian, S. Sullivan, B. Sankaran, R.W. Edom, N. Weng, D. Sharkey, Development and Validation of an LCMS/MS Based Method for Quantification of 25 Hydroxyvitamin D2 and 25 Hydroxyvitamin D3 in Human Serum and Plasma, Journal of Chromatography B (2014), http://dx.doi.org/10.1016/j.jchromb.2014.05.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Development and Validation of an LC-MS/MS Based Method for Quantification of 25 Hydroxyvitamin D2 and 25 Hydroxyvitamin D3 in Human Serum and Plasma
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Richard W. Edomb, Naidong Wengb, and David Sharkeya
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Stanley (Weihua) Zhanga*, Wenying Jianb, Sheryl Sullivana, Banu Sankarana,
a
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Ortho Clinical Diagnostics, Johnson & Johnson, 1001 Route 202 North, Raritan,
New Jersey 08869, United States b
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Janssen Research and Development, Johnson & Johnson, 1400 McKean Road,
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Spring House, Pennsylvania 19477, United States
* Corresponding author at: Ortho Clinical Diagnostics, Johnson & Johnson, 1001
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Route 202 North, Raritan, New Jersey 08869, United States. Tel.: +1 908 218
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8735. E-mail address: [email protected] (Stanley Weihua Zhang).
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Abstract Vitamin D deficiency is increasing in the general population and has become a serious public health risk globally. As a reliable clinical indicator of
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vitamin status, 25 hydroxyvitamin D (25(OH)D) has been measured by various
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methods. However, the accuracy of these measurements has been the subject of considerable debate. Here, we report the development and validation of a liquid
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chromatography-triple quadrupole mass spectrometry based method for the
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quantification of 25(OH)D2 and 25(OH)D3 in human serum and plasma samples. Samples were first processed by protein precipitation to release the analytes
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from the vitamin D binding protein (DBP), followed by a liquid-liquid extraction procedure. Analysis was performed on an LC-MS/MS system which utilized an
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AB Sciex API 3000 mass spectrometer. A six point calibration curve ranging from
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2.5 - 100 ng/mL was established for both 25(OH)D2 and 25(OH)D3. A complete
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method validation was conducted, including intra- and inter-assay accuracy and precision, LLOQ, dilution QC, specificity, recovery, matrix effect, and a thorough stability profile of stock solutions and QC samples. Matching samples of serum and plasma (containing either heparin or EDTA anticoagulant) generated from the same blood samples were tested, and no significant differences in 25(OH)D2 and 25(OH)D3 concentrations were found in these sample matrices. In method comparison, we analyzed 10 serum samples obtained from the Vitamin D External Quality Assessment Scheme (DEQAS), and the total 25(OH)D concentrations measured by our method were very close to the LC-MS/MS Method Mean values provided by DEQAS (average 0.17% bias, R² = 0.99). 2
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However, comparison with the DiaSorin Liaison 25(OH)D TOTAL Assay demonstrated limited correlation between these two methods (R² = 0.54). In general, concentrations measured by our LC-MS/MS method were roughly 9%
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higher than those measured by the DiaSorin Liaison assay. The correlation with
DiaSorin Liaison measurement was better for samples in the lower concentration
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range. In summary, we developed and validated an LC-MS/MS based method
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that can be reliably applied in routine quantification of 25(OH)D2 and 25(OH)D3 in human serum and plasma samples. This method is not suitable for pediatric
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determinations due to the potential interference of 3-epi 25(OH)D3.
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Keywords
Abbreviations
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comparison, stability.
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25-hydroxyvitamin D, LC-MS/MS, validation, matrix comparison, method
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25(OH)D, 25-hydroxyvitamin D; 25(OH)D2, 25-hydroxyvitamin D2;
25(OH)D3, 25-hydroxyvitamin D3; 1,25(OH)2D, 1,25-dihydroxyvitamin D; vitamin D binding protein (DBP); RIA, radioimmunoassay; ELISA, enzyme-linked immunosorbent assay; HPLC, high performance liquid chromatography; LCMS/MS, liquid chromatography-tandem mass spectrometry; DEQAS, the Vitamin D External Quality Assessment Scheme; QC, Quality control; MRM, Multiple reaction monitoring; BSA, Bovine serum albumin; PBS, phosphate-buffered saline; CV, coefficients of variation; LLOQ, lower limit of quantification; EDTA, ethylenediaminetetraacetic acid; CLIA, chemiluminescence immunoassay. 3
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1. Introduction Vitamin D, including vitamin D2 and vitamin D3, is essential for the bone health of humans. Clinical trials have shown that oral vitamin D supplements
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helped to prevent fractures [1]. Vitamin D deficiency has also been implicated in
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various cancers [2], cardiovascular diseases [3], and autoimmune diseases [4].
Vitamin D deficiency is increasing in the general population and is considered an
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important public health problem. The Institute of Medicine (IOM) released new
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dietary reference intakes for calcium and vitamin D in 2011, defining four categories of vitamin D status based on serum 25-hydroxyvitamin D (25(OH)D):
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(i) risk of deficiency, (ii) risk of inadequacy, (iii) sufficiency, and (iv) above which there may be reason for concern [5]. Based on these four categories, the US
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Centers for Disease Control and Prevention (CDC) reported that 24% of the US
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population of one year and older were at risk of inadequacy, and 8% were at risk
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of deficiency in 2001-2006 [6]. Experts in the field have argued that higher concentration levels should be set to define inadequacy and deficiency, thus making a higher percentage of the population fall into these categories [7]. In humans, vitamin D3 is produced from its precursor 7-
dehydrocholesterol during exposure to ultraviolet rays in sunlight, or it can be consumed in the diet. The human body does not make vitamin D2, and the normally low level of vitamin D2 in humans is from dietary intake. Vitamin D is biologically inactive, and it requires enzymatic conversion to produce active metabolites. As shown in Figure 1, Vitamin D is converted to 25(OH)D, the major circulating form of vitamin D, and then to 1,25-dihydroxyvitamin D (1,25(OH)2D), 4
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the active form of vitamin D, by enzymes in the liver and kidney [8]. Although 1,25(OH)2D is the active metabolite of vitamin D, serum levels of 1,25(OH)2D do not reflect the body’s storage and are not useful for determining vitamin D status.
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Instead, 25(OH)D concentration is accepted as a reliable clinical indicator of vitamin D status [9].
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Traditionally, 25(OH)D has been measured by radioimmunoassay (RIA),
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enzyme-linked immunosorbent assay (ELISA) and high performance liquid chromatography (HPLC). In recent years, liquid chromatography-tandem mass
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spectrometry (LC-MS/MS) has become a popular method based on its superior specificity and ability to measure 25(OH)D2 and 25(OH)D3 separately but
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simultaneously. The method validation in publications based on LC-MS/MS
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techniques has mostly focused on key elements of the assays, such as linearity,
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accuracy and precision, specificity, and recovery, etc., but missed other important items such as stabilities and matrix effects. Here, we report a complete
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validation of an LC-MS/MS based method for the quantification of 25(OH)D2 and 25(OH)D3 in human serum and plasma. To address the issue of lack of human serum or plasma samples that had no endogenous 25(OH)D3, we used 5% bovine serum albumin (BSA) as a surrogate matrix for calibration curves as well as lower limit of quantification (LLOQ) quality control (QC) samples, and a horse serum pool, which contained marginal levels of vitamin D as a surrogate matrix for low level QC (LQC) samples. Our validation included detailed experiments on stability of test samples, stock solutions, and processed samples, as well as comparisons between matching serum, heparin, and EDTA plasma samples. 5
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This method was compared with other LC-MS/MS methods based on measurement of 10 samples from the Vitamin D External Quality Assessment Scheme (DEQAS, http://deqas.kpmd.co.uk) [10], and to the widely used DiaSorin
applications (http://www.diasorin.com/en/node/8945/) [11].
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2. Material and Methods
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Liaison 25(OH)D TOTAL Assay, an immunoassay approved by FDA for clinical
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2.1. Chemicals and Reagents
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The vitamin D related chemicals were purchased from the following sources: Vitamin D2 and vitamin D3, Sigma-Aldrich (St. Louis, MO); 25(OH)D2,
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BePharm Ltd. (Shanghai, China); 25(OH)D3 and 1,25(OH)2D3, Toronto Research Chemicals, Inc. (Toronto, Ontario, Canada); 1,25(OH)2D2 and
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24,25(OH)2D3, Santa Cruz Biotechnology (Dallas, Texas). 3-epi 25(OH)D3,
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internal standards [2H]3-25(OH)D2 and [2H]3-25(OH)D3, IsoSciences (King of
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Prussia, PA). Other chemicals and reagents were purchased from the following sources: HPLC grade methanol and acetonitrile, EMD Chemicals (Philadelphia, PA); heptane, Mallinckrodt Chemicals (Hazelwood, MO); ammonium acetate, Fluka (St. Louis, MO); Dulbecco's phosphate-buffered saline (PBS), Mediatech, Inc. (Manassas, VA); bovine serum albumin (BSA), Sigma-Aldrich (St. Louis, MO).
2.2. Serum and Plasma samples Normal human serum samples and horse serum samples were purchased from Bioreclamation, Inc. (Hicksville, NY) and were used for making quality 6
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control (QC) samples. For matrix comparison, matching serum, heparin, and K2EDTA plasma samples from 25 healthy donors were prepared from fresh blood samples collected in a donor room at Ortho Clinical Diagnostics. For method
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comparison to the DiaSorin Liaison 25(OH)D TOTAL Assay, human serum
samples were either collected in a donor room at Ortho Clinical Diagnostics or
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purchased from the following suppliers: Vitrologic (Charleston, SC),
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Bioreclamation, Inc. (Hicksville, NY), Complex Antibodies, Inc. (Margate, FL), and New York Biologics, Inc. (Southampton, New York). Ten DEQAS serum
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samples were obtained from the DEQAS organization.
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2.3. Sample Preparation
Thermally equilibrate calibration standard samples, QCs, study samples,
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blank samples (horse serum), and solutions at room temperature. Mix 200 µL of
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each sample with 25 µL of internal standards at 320 ng/mL in methanol for both
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[2H]3-25(OH)D2 and [2H]3-25(OH)D2 in a 13x100 mm glass culture tube and equilibrate for 5 min. Add 200 µL of methanol to each tube, vortex for 10 seconds, and equilibrate for 5 minutes. Add 2 mL of heptane to each tube and vortex the tubes on a Multi-Tube Vortexer at 1600 rpm for 10 minutes. Centrifuge the tubes at 3220 rcf for 5 minutes. Transfer 1.6 mL of the top organic layer to a 2-mL 96well plate and evaporate to dryness under nitrogen at room temperature. Add 100 µL of reconstitution solution (2 mM ammonium acetate, 0.1% formic acid in 80/20: methanol/water (v/v)) to each well. Vortex the plate on a plate shaker at 1000 rpm for 2 minutes, and subject the samples to LC/MS/MS analysis. 7
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2.4. LC-MS/MS LC-MS/MS was performed on a Shimadzu HPLC system, which consisted of Shimadzu LC10AD pumps and a SIL-HTC autosampler (Kyoto, Japan),
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coupled to an AB Sciex API 3000 triple-quadrupole mass spectrometer
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(Framingham, MA). Ten μL of each sample was injected on a Zorbax SB-C18,
3.5 µm, 2.1x50 mm HPLC column (Agilent, Santa Clara, CA). The mobile phases
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were: (A) 2 mM ammonium acetate, 0.1% formic acid in water, and (B) 2 mM
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ammonium acetate, 0.1% formic acid in methanol. The needle rinse solvent was methanol. The HPLC flow rate was set at 0.3 mL/min. The gradient elution
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started at 80% mobile phase B for the first 0.1 min, was ramped linearly to 95% B in 0.9 min, held at 95% B for 4 min, and then returned to 80% B in 0.1 min. The
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column was re-equilibrated with 80% B for 2.9 min prior to the next injection. The
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LC flow was diverted to waste before 0.1 min and after 5 min of each run. The
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API 3000 was set to positive ionization mode, with curtain gas set at 10 psi, nebulizer gas at 10 psi, IonSpray™ voltage at 3000 V, ion source temperature at 450C and CAD gas at a nominal setting of 6. The MRM transitions for the analytes and their internal standards were: 25(OH)D2, m/z 413.3→355.2; [2H]325(OH)D2, m/z 416.3→358.2; 25(OH)D3, m/z 401.3→365.2; and [2H]3-25(OH)D3, m/z 404.3→368.2. For 25(OH)D2 and [2H]3-25(OH)D2, the declustering potential (DP), focusing potential (FP), and collision energy (CE) were set at 35 V, 140 V, and 35 eV, respectively. For 25(OH)D3 and [2H]3-25(OH)D3, the DP was set at 30 V and the FP was set at 120 V. The CE was set at 32 eV for 25(OH)D3 and
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30 eV for [2H]3-25(OH)D3. For all four channels, the entrance potential (EP) was set at 10 V and the collision cell exit potential (CXP) was set at 20 V. 2.5. Method Validation
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Co-mingled calibration standards were prepared at levels of 2.5/2.5,
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5.0/5.0, 10/10, 25/25, 50/50, and 100/100 ng/mL (25(OH)D2 / 25(OH)D3) in 5% BSA in PBS. A co-mingled internal standard working solution was prepared in
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methanol containing [2H]3-25(OH)D2 and [2H]3-25(OH)D3, each at a
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concentration of 320 ng/mL. All stock solutions were prepared and stored in amber vials for protection from the light. Calibration curves were constructed by
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plotting the chromatographic peak area ratios of 25(OH)D2 and 25(OH)D3 to their internal standards versus the corresponding concentrations, and fitting the
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data using linear regression with a 1/x2 weighting factor. QC samples at five
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levels (LLOQ, LQC, MQC, HQC, and dilution QC) were prepared in either 5%
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BSA, a pooled horse serum sample, or a pooled human serum sample. The BSA lot we used had previously been shown to devoid any 25(OH)D contamination. To determine the endogenous levels of 25(OH)D2 and 25(OH)D3 in the pooled horse serum sample and the pooled human serum sample, six replicate samples from each pool were analyzed using a calibration curve and the resulting concentration readings were averaged for each pool. Additional reference standards were then spiked in the pools to reach the desired QC concentrations. LLOQ QC was prepared in 5% BSA. LQC was prepared in the pooled horse serum, which contains minimal endogenous 25(OH)D. MQC, HQC, and dilution QC were prepared in the pooled human serum. 9
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2.5.1. Intra-assay Precision and Accuracy Six replicates each of three QC levels (low, medium, and high) were analyzed in a single run to assess intra-assay precision. The means, standard
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deviations, coefficients of variation (%CV) and mean percent deviation of the
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back-calculated values (% Bias) were calculated at each QC concentration.
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2.5.2. Inter-assay Precision and Accuracy
Three calibration curves were run over three analysis days. Each
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calibration curve included six QCs at three levels (low, medium, and high). The calibration curves were each regressed separately. For each calibration curve,
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the slope, intercept, coefficient of determination, and individual percent deviations were calculated. The mean, standard deviation, %CV, and mean
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percent deviation of the back-calculated values (% Bias) were calculated at each
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2.5.3. LLOQ
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standard and QC concentration.
The accuracy and precision of the LLOQ was assessed through the
analysis of replicate QCs at the LLOQ concentration during the Inter-Assay Precision and Accuracy experiment. 2.5.4. Dilution QC
Six replicates of dilution QC samples were diluted to 50 ng/mL with 5% BSA and assayed as per the method procedure. The mean % deviation from nominal value and %CV were calculated. 10
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2.5.5. Specificity Six serum samples from a same lot were spiked with one of the six different 25(OH)D analogs (vitamin D2, vitamin D3, 1,25(OH)2D2, 1,25(OH)2D3,
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24,25(OH)2D3, and 3-epi-25(OH)D3) at 200 ng/mL. The concentrations of
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25(OH)D2 and 25(OH)D3 before and after spiking were measured in singlet and
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compared for detection of potential interference. 2.5.6. Recovery
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Recovery of 25(OH)D2 during the sample preparation process was determined at three QC concentration levels (low, medium, and high). The
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resulting chromatographic peak areas from extracted samples were compared
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against those post-spiked to an extracted normal human serum pool which
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contained an insignificant level of pre-existing 25(OH)D2. Internal standards were added after extraction to offset any variations from LC-MS/MS performance
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for both pre- and post-spiked samples. Six replicates at each concentration were assayed. The mean % recovery and %CV were calculated at each concentration. Recovery of 25(OH)D3 was determined exactly as that of 25(OH)D2
except that two QC concentration levels were used (medium and high) due to the lack of human serum samples with very low concentrations of 25(OH)D3. To account for the endogenous 25(OH)D3 concentration in the normal human serum pool, the following equation was solved to obtain the recovery value X: (Endogenous concentration + Concentration of spike)*X%
Peak Area ratio of pre-spike/internal standard =
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Endogenous concentration *X% + Concentration of spike
Peak Area of post-spike/internal standard
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2.5.7. Matrix Effect Absolute matrix effect was determined by comparing instrument signals of
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analytes post-spiked in extracted biological matrix to those in neat solvent at the
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same level. At first, six different lots of human serum samples were fully
processed in triplicate and analyzed to obtain the average concentration of
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endogenous 25(OH)D2 and 25(OH)D3 in each sample. Then, the same six lots of human serum samples (without spiking of internal standard) were extracted in
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duplicate, evaporated to dryness, and reconstituted with analytes to reach either the medium (50 ng/mL) or the high level (80 ng/mL) QC concentration along with
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internal standards. The analytes needed for the reconstitution was equal to the
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desired total analytes in a reconstituted sample minus the endogenous analytes
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times the recovery rate determined in the “Recovery” experiment. The measurement of these reconstituted samples provided instrument signals of a known amount of 25(OH)D2 and 25(OH)D3 in the presence of human serum matrix. Finally, an identical medium or high level of the 25(OH)D2 and 25(OH)D3 was added to the reconstitution solution (no matrix) in six replicate samples. These samples were analyzed exactly as the above serum samples to obtain the mean response of the analytes in the absence of matrix. The absolute matrix factor was calculated for the analytes at each concentration as the ratio of peak response in presence of matrix ions to mean peak response in absence of matrix ions. 12
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Relative matrix effects were calculated as ratios of peak intensities of analytes over corresponding internal standards in the presence of human serum
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matrix. 2.5.8. Matrix Comparison
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Matching serum and plasma samples (containing either heparin or K2-
EDTA anticoagulant) generated from the same blood samples from 25 healthy
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donors were prepared from fresh blood collected in a donor room at Ortho
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Clinical Diagnostics. 25(OH)D2 and 25(OH)D3 concentrations from these three different sample matrices from each individual were determined and compared.
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2.5.9. Stock Solution Stability
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2.5.9.1. Room Temperature Stability
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Room temperature stability was evaluated by comparing the instrument response of a portion of the stock solution stored at room temperature for six
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hours against the remainder of the solution stored in a -20°C freezer. Six replicate injections at a single concentration (10 ng/mL) for both 25(OH)D2 and 25(OH)D3 were made for each solution. 2.5.9.2. Freezer Stability
Frozen stock solutions were stored in a -20°C freezer for six months. A
solution of 25(OH)D2 and 25(OH)D3 at 10 ng/mL each was prepared from the stock solutions. Six replicate injections of the solution were made and the peak intensities were compared against those obtained from a freshly prepared stock solution from a new weighing. 13
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2.5.10. QC Sample Stability 2.5.10.1. Bench-top Stability Six replicates of QC samples at two concentration levels (low and high)
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were allowed to thaw and remain at room temperature for 24 hours prior to
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preparation and analysis. Mean concentrations were calculated and compared
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against the nominal concentrations. 2.5.10.2. Freezer Stability
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Freezer stability of the study samples was evaluated by analyzing six replicates of QC samples at three concentration levels (low, medium, and high)
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that were stored in a -20°C freezer for 265 days. A freshly prepared calibration
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concentrations.
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curve was used. Calculated concentrations were compared to nominal target
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2.5.10.3. Freeze/thaw Stability
The effect of freeze/thaw stability was tested for four cycles using six
replicates of QC samples at two concentration levels (low and high). For the first freeze/thaw cycle, the QC samples were stored at -20°C for 24 hours before being allowed to thaw at room temperature. For the second, third, and fourth cycles, the QC samples were stored at -20°C for a minimum of 12 hours before being allowed to thaw. The samples were out of the freezer for at least 1 hour between each cycle. The mean concentrations following the fourth cycle were determined for each QC concentration.
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2.5.11. Processed Sample/Autosampler Stability and Re-injection Reproducibility To evaluate processed sample/autosampler stability, and re-injection
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reproducibility, a complete calibration curve with six replicates of QC samples at three concentration levels (low, medium, and high) were prepared and injected.
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At the completion of the analysis, the samples were stored in the autosampler.
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To determine re-injection reproducibility, the calibration curve and QC samples were re-injected at 12 hours following the completion of the original run. The
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processed sample/autosampler stability was determined after the samples had been stored in the autosampler for 72 hours. A fresh calibration curve was
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prepared, and the original QC samples were re-injected with the freshly prepared
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2.6. Method Comparison
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calibration curve.
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2.6.1. Comparison with LC-MS/MS Method Mean of Total 25(OH)D in DEQAS Samples
Ten blind serum samples from the Vitamin D External Quality Assessment
Scheme (DEQAS) were analyzed. The measured 25(OH)D2 and 25(OH)D3 concentrations were added together as total 25(OH)D concentration for each sample and compared with the corresponding LC-MS/MS Method Mean total value provided by DEQAS. 2.6.2. Comparison with DiaSorin Liaison 25(OH)D TOTAL Assay
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A total of 145 normal human serum samples of male and female, age 10 and older, and representing different combinations of low, medium, and high concentrations of 25(OH)D2 and 25(OH)D3 were tested in singlet. The same set
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of samples was also analyzed by the chemiluminescence immunoassay (CLIA) based DiaSorin Liaison 25(OH)D TOTAL Assay at ACM Medical Laboratories
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(Rochester, NY). The total 25(OH)D concentrations measured by both methods
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were compared using linear regression analysis.
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3. Results and Discussion 3.1. Method Development
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We originally tested protein precipitation as the sample preparation method, but we encountered strong matrix effects and decreased sensitivity with
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the API 3000 mass spectrometer. Therefore, we modified this method by adding
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a liquid-liquid extraction step to clean up the samples following protein
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precipitation, which was still necessary for releasing 25(OH)D from the vitamin D binding protein (DBP). We also observed that measured 25(OH)D2 and 25(OH)D3 concentrations were higher when internal standards were added before protein precipitation (followed by a five minute equilibration) than when they were directly added in the protein precipitation solvent. We speculate that equilibration of the internal standards in the samples allowed them to bind to vitamin D binding protein, and therefore, track the recovery of the analytes more accurately. Sample preparation details such as this are very important, and different ways of handling them may contribute to the reported variability of LCMS/MS assays [12]. Figure 2 (A) and (B) show typical mass chromatograms from 16
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an extracted medium concentration level (50 ng/mL) QC sample for 25(OH)D2 and 25(OH)D3, and a normal human serum sample, respectively.
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3.2. Quantification Range, Linearity, Accuracy, and Precision It is very difficult to obtain human serum or plasma samples that have no
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endogenous 25(OH)D3, and therefore, 5% BSA was used as a surrogate matrix
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for calibration curves. In this case, it is important to fully evaluate the method using QC samples prepared at least at one concentration level using authentic
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matrix to demonstrate the absence of potential bias introduced by using surrogate matrix [13]. In the current method, the LLOQ QC was prepared in 5%
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BSA. A horse serum pool, which contained marginal levels of vitamin D, was screened for its baseline concentration prior to being spiked with a known
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amount of analytes to reach the expected LQC concentration. Pooled human
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serum was spiked on top of the measured endogenous levels to reach the
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designated MQC, HQC and dilution QC concentrations. Using this approach, most of the QC levels were prepared in the expected biological matrix for comprehensive evaluation of the method. Linearity was evaluated by constructing calibration curves over the
concentration range of 2.5-100 ng/mL for both 25(OH)D2 and 25(OH)D3. The r values for both analytes were ≥ 0.999. The LLOQ, defined as the lowest concentration that gave a signal-to-noise ratio of 10, was validated to be 2.5 ng/mL for both 25(OH)D2 and 25(OH)D3. To account for potentially elevated levels of 25(OH)D2 and 25(OH)D3 when vitamin D supplements or drugs were 17
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taken, a dilution QC sample at 250 ng/mL for each analyte was validated to produce acceptable accuracy and precision when it was diluted to 50 ng/mL with 5% BSA for sample analysis.
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Method accuracy and precision was evaluated using QC samples at low,
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medium, and high concentrations. The intra-assay variability was evaluated from six replicate QC’s at three concentrations determined in a single run. The %CV
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was 4.3 for all levels of 25(OH)D2 and 3.2 for all levels of 25(OH)D3. The
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mean percentage deviations of the back-calculated concentrations were within 7.4% of the nominal concentrations for all levels of 25(OH)D2, and within 5.0%
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of the nominal concentrations for all levels of 25(OH)D3. The inter-assay variability was obtained from 18 replicate QC’s at three concentrations analyzed
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over three different days. The mean percent deviation of the back-calculated
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concentrations were within 7.05% of the nominal concentrations for all levels of
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25(OH)D2 and within 6.08% of the nominal concentrations for all levels of 25(OH)D3. The %CVs were within 5.03 for all levels of 25(OH)D2, and within 4.42 for all levels of 25(OH)D3. The intra- and inter-assay accuracy and precision results were summarized in Table 1. 3.3. Specificity
To evaluate specificity of the assay, we tested six different 25(OH)D analogs (vitamin D2, vitamin D3, 1,25(OH)2D2, 1,25(OH)2D3, 24,25(OH)2D3, and 3-epi-25(OH)D3). Six serum samples from a same lot were spiked with one of the analogs at 200 ng/mL. The concentrations of 25(OH)D2 and 25(OH)D3 18
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before and after spiking were measured. If there was any interference from these analogs, the concentrations of 25(OH)D2 and/or 25(OH)D3 would have changed after spiking. Our data showed that except for 3-epi-25(OH)D3, none of the other
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analogs showed any significant interference to the quantification of 25(OH)D2 and 25(OH)D3. 3-epi-25(OH)D3 shared the identical MRM transition to
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25(OH)D3, and it caused interference when it was not chromatographically
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separated. However, it has been shown to be present solely in infants [14]. Although the current method could not differentiate 3-epi-25(OH)D3 from
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25(OH)D3, we did not consider further separation since this assay was not designed for an infant population.
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3.4. Recovery
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Recovery of 25(OH)D2 during the sample preparation process was
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determined using the low, medium, and high QC concentrations. The recovery
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for all concentration levels of 25(OH)D2 was ≥ 72% with a %CV ≤ 7.05. The recovery of 25(OH)D3 was only examined at the medium and high QC concentrations (Section 2.5.6). The endogenous 25(OH)D3 in the normal human serum pool used for making QC samples was accounted for during the recovery calculation. The recovery for both levels of 25(OH)D3 was ≥ 62% with a %CV ≤ 3.03. Even though the recovery for both analytes was not optimal, the acceptable performance of the QC samples indicated that the stable isotope labeled internal standards fully tracked the analytes. 3.5. Matrix Effects and Matrix Comparison 19
Page 19 of 38
Analyte signal suppression or enhancement during ionization due to biological matrix was evaluated by determining the matrix effects in six serum sample lots. Since all six serum lots had certain levels of endogenous analytes,
ip t
only the medium and high QC concentrations were examined. The serum samples were analyzed first to determine the endogenous 25(OH)D2 and
cr
25(OH)D3 concentrations. The endogenous 25(OH)D in each sample, multiplied
us
by the recovery rate obtained from the Recovery experiments, was the
endogenous 25(OH)D recovered during extraction. Extra 25(OH)D was spiked
an
into these extracted samples to reach the medium and high QC concentrations. The absolute matrix factor was calculated as the ratio of the peak response in
M
presence of matrix ions to the mean peak response in absence of matrix ions.
d
For 25(OH)D2, the absolute matrix factor was 0.47 (%CV 45.1) for the medium
te
level concentration and 0.13 (%CV 11.9) for the high level concentration. For 25(OH)D3, the absolute matrix factor was 0.44 (%CV 39.0) for the medium level
Ac ce p
concentration and 0.13 (%CV 18.2) for the high level concentration. Except for the high level concentration of 25(OH)D2, all results of the
absolute matrix effect experiments for 25(OH)D2 and 25(OH)D3 did not meet our acceptance criteria (%CV ≤ 15). Therefore, we evaluated relative matrix effects using peak area ratios of the analytes to their corresponding internal standards. The %CV observed for 25(OH)D2 was 1.5 for the medium level concentration and 3.5 for the high level concentration. The %CV observed for 25(OH)D3 was 1.5 for the medium level concentration and 3.0 for the high level concentration. Relative matrix effect results for both 25(OH)D2 and 25(OH)D3 thus met our 20
Page 20 of 38
acceptance criteria, indicating that the stable isotope labeled internal standards compensated for the ion suppression.
ip t
The current assay was designed to be used with different sample matrices, such as serum and plasma. One major concern was that different
cr
anticoagulants might affect assay performance, as experienced by some LC-
MS/MS test labs [12]. To evaluate potential effects of different sample matrices
us
on assay performance, matching serum, heparin, and K2-EDTA plasma was
an
generated from the same blood samples from 25 individuals. The samples were analyzed and compared, and the results were summarized in Table 2. Only 3 out
M
of 25 donors had 25(OH)D2 readings above the LLOQ (2.5 ng/mL), and the differences between heparin and EDTA plasma to serum were all smaller than
d
15%. For heparin plasma, the average difference from serum was 6.95%, and
te
for EDTA plasma it was -2.19%. For 25(OH)D3, when heparin plasma was
Ac ce p
compared to serum, 3 out of 25 donors showed a difference of >15%, but the average difference for all samples was only 3.37%. When EDTA plasma was compared to serum, only 1 out of 25 donors showed a difference of >15%, and the average difference for all samples was -3.84%. These results demonstrated that different sample matrices, serum, heparin and K2-EDTA plasma, did not cause significant variation in 25(OH)D2 and 25(OH)D3 quantification. 3.6. Stock Solution and Study Sample Stability Even though 25(OH)D2 and 25(OH)D3 in human plasma and serum has been known to be very stable, very few publications have described its complete 21
Page 21 of 38
stability profile [15]. Therefore, we conducted extensive studies to determine the stability of both analyte stock solutions and study samples, and the results were summarized in Table 3. The stock solutions for 25(OH)D2 and 25(OH)D3 were
ip t
found to be stable for 6 hours at room temperature and for 181 days in a -20°C
freezer (stability beyond 6 hours at room temperature and beyond 181 days at -
cr
20°C was not tested).
us
25(OH)D2 and 25(OH)D3 in human serum samples were found to be
an
stable at room temperature for 24 hours, and for 265 days in a -20°C freezer (stability of longer time was not tested). 25(OH)D2 and 25(OH)D3 in human
M
serum samples was stable for four freeze/thaw cycles.
The re-injection reproducibility and processed sample/autosampler
d
stability was examined with a complete calibration curve with six replicates of
te
three QC concentration levels (low, medium, and high). To determine re-injection
Ac ce p
reproducibility, the calibration curve and QC samples were re-injected following the completion of the original run. It was found that the mean concentration of each level of the QC samples was within 15% of their respective nominal value after re-injection. To determine processed sample/autosampler stability, the original QC samples were re-injected with a freshly prepared calibration curve after these samples had been stored in an autosampler at room temperature for 72 hours. 25(OH)D2 and 25(OH)D3 were both found to be stable. 3.7. Method Comparison
22
Page 22 of 38
The concentrations of total 25(OH)D in human blood measured by different testing methods can vary dramatically [16]. Since almost all human blood samples contain a certain level of 25(OH)D, there is no true blank matrix to
ip t
make QC or reference samples, and it is impossible to know if any individual
25(OH)D measurement is the true value. In order to compare the accuracy of our
cr
method to that of other LC-MS/MS methods, we took advantage of unprocessed
us
human serum samples distributed by DEQAS, which was incorporated in 1989 to ensure the analytical reliability of vitamin D assays. DEQAS samples were non-
an
spiked, normal human serum pools. DEQAS provided the LC-MS/MS Method Mean values of total 25(OH)D for each sample, which was the average test value
M
from at least one hundred different LC-MS/MS test sites [10]. Ten samples
d
(sample number 386-395) obtained from DEQAS were analyzed in singlet in a
te
blind test. The measured 25(OH)D2 and 25(OH)D3 concentrations were added together as the total 25(OH)D concentration for each sample and were compared
Ac ce p
with the corresponding LC-MS/MS Method Mean total value provided by DEQAS, as shown in Table 4 and Figure 3. The data demonstrated that our measurements were very close to DEQAS LC-MS/MS Method Mean values. The average difference from DEQAS LC-MS/MS Method Mean was 0.17%, with the largest difference at -16.87%. The regression analysis formula was LC-MS/MS = 1.0503 x DEQAS LC-MS/MS Method Mean – 0.6845, and the R2 was 0.9884, indicating excellent correlation. We also compared our method with the DiaSorin Liaison 25(OH)D TOTAL Assay, a chemiluminescence immunoassay (CLIA ) approved by FDA for 23
Page 23 of 38
measurement of total 25(OH)D concentrations in clinical applications. The DiaSorin Liaison 25(OH)D TOTAL Assay is the most popular assay in hospitals and commercial clinical laboratories. In the DEQAS July 2009 distribution report,
ip t
the DiaSorin Liaison assay was used by over 36% of about 600 laboratories [12]. Therefore, it was essential to compare the performance of our LC-MS/MS
cr
method to that of the DiaSorin Liaison assay. A total of 145 normal human serum
us
samples, representing different combinations of low, medium, and high
concentrations of 25(OH)D2 and 25(OH)D3 were tested in singlet by the LC-
an
MS/MS method, and by the DiaSorin Liaison 25(OH)D TOTAL Assay, which was performed at ACM Medical Laboratories (Rochester, NY). The total 25(OH)D
M
concentrations measured by both methods were analyzed by linear regression
d
analysis, as shown in Figure 4 (a). The resulting formula was LC-MS/MS =
te
1.0850 x DiaSorin – 3.3274, which indicated that, in general, concentrations measured by our LC-MS/MS method were roughly 9% higher than those
Ac ce p
measured by the DiaSorin Liaison assay. The R2 of 0.5439 suggested that there was limited correlation between these two methods. This result was in agreement with other published data when LC-MS/MS methods and the DiaSorin Liaison assay were compared [12, 17]. When the data were examined more closely, it appeared that there was a
better correlation between these two methods for samples of lower 25(OH)D concentrations, especially those with DiaSorin Liaison measurements lower than 60 ng/mL or so. A separate regression analysis was thus performed on all samples with measurements lower than 60 ng/mL in the DiaSorin Liaison assay 24
Page 24 of 38
(total 128 samples), and the result is shown in Figure 4 (b). The resulting formula was LC-MS/MS = 1.1953 x DiaSorin + 0.6319, and the R2 was 0.8181. Even though the LC-MS/MS readings for these samples were still roughly 20% higher
ip t
than those of the DiaSorin Liaison assay, the correlation was improved relative to that of the full sample set. Since the majority of people in the general population
cr
have total 25(OH)D concentrations less than 60 ng/mL (the proposed possibly
us
harmful level by the Institute of Medicine (IOM) was 50 ng/mL [5]), our results suggested that these two methods should correlate quite well with each other in
an
clinical tests within the general population.
M
There is no clear explanation as to why DiaSorin Liaison assays have generally provided lower 25(OH)D measurements than LC-MS/MS based
d
methods. In LC-MS/MS based methods, native analytes are normally tracked by
te
stable-isotope-labeled internal standards having identical physicochemical
Ac ce p
behaviors. Such methods should, therefore, yield the highest attainable analytical accuracy, and the LC-MS/MS based methods are widely considered the gold standard for 25(OH)D quantification[18]. One possible reason for lower DiaSorin Liaison assay readings is the potential low recovery of 25(OH)D from sample preparation. Instead of organic solvent extraction used in most LC-MS/MS based methods, the DiaSorin Liaison assay relies on pH change and/or blocking agents that liberate the 25(OH)D from the vitamin D binding protein (DBP). Any possible loss of target that occurs during the extraction method will result in lower recovery without proper internal standard for correction. Indeed, a recent report showed that an inverse relationship between the DBP concentrations and relative 25
Page 25 of 38
recovery was observed in the DiaSorin Liaison assay when it was compared to an LC-MS/MS based method [19]. In another report, low recovery was observed for exogenously added 25(OH)D in human serum samples, and it was suspected
ip t
that the exogenous 25(OH)D did not distribute properly on the DBP [20]. More
cause of low recovery in the DiaSorin Liaison assay.
us
4. Conclusion
cr
experiments are needed to examine whether the extraction method is the true
an
We have developed an LC-MS/MS based method for quantification of 25(OH)D2 and 25(OH)D3 in human serum and plasma and conducted a
M
validation covering accuracy and precision, LLOQ, dilution QC, specificity, recovery, matrix effect, and a thorough stability profile of stock solutions and QC
d
samples. The extraction method, which consisted of a combination of methanol
te
precipitation and liquid-liquid extraction by heptane, produced a relatively clean
Ac ce p
extract compatible with most triple-quadrupole instruments. Matrix comparisons between matching serum, heparin, and EDTA plasma samples demonstrated that this LC-MS/MS method was suitable for these sample matrices without producing bias among the results. This assay should not be used for pediatric determinations without further chromatographic development due to the potential interference of 3-epi 25(OH)D3. Our method was shown to have accuracy that closely followed the LCMS/MS Method Mean when 10 DEQAS samples were tested in blind. On the other hand, in a test of 145 normal human serum samples from males and 26
Page 26 of 38
females of age 10 and up, the total 25(OH)D concentrations obtained by our LCMS/MS assay were, in general, about 9% higher than those obtained by the DiaSorin Liaison 25(OH)D TOTAL Assay, and there was limited correlation (R2 =
ip t
0.5439) between these two methods. The correlation was better for samples with
cr
DiaSorin Liaison measurements lower than 60 ng/mL (R2 = 0.8181).
us
Acknowledgements
We wish to thank Ms. Susan Chazan and Dr. Lucius (Tad) Fox for their
an
inputs in method validation; Dr. Wensheng Lang for his assistance in the early
Ac ce p
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d
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stage of the project.
27
Page 27 of 38
References:
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te
d
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an
us
cr
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[1] H.A. Bischoff-Ferrari, W.C. Willett, J.B. Wong, A.E. Stuck, H.B. Staehelin, E.J. Orav, A. Thoma, D.P. Kiel, J. Henschkowski, Arch. Intern. Med., 169 (2009) 551-561. [2] C. Buttigliero, C. Monagheddu, P. Petroni, A. Saini, L. Dogliotti, G. Ciccone, A. Berruti, Oncologist, 16 (2011) 1215-1227. [3] C. McGreevy, D. Williams, Ann. Intern. Med., 155 (2011) 820-826. [4] A. Ascherio, K.L. Munger, K.C. Simon, Lancet Neurol., 9 (2010) 599-612. [5] A.C. Ross, C.L. Taylor, A.L. Yaktine, H.B. Del Valle, Institute of Medicine of National Academies, The National Academies Press, Washington, DC., (2010). [6] A.C. Looker, C.L. Johnson, D.A. Lacher, C. Pfeiffer, R.L. Schleicher, C.T. Sempos, NCHS Data Brief, 59 (2011). [7] R.P. Heaney, M.F. Holick, J. Bone Miner. Res., 26 (2011) 455-457. [8] S. Christakos, D.V. Ajibade, P. Dhawan, A.J. Fechner, L.J. Mady, Endocrinol. Metab. Clin. North Am., 39 (2010) 243-253, table of contents. [9] K.M. Seamans, K.D. Cashman, Am. J. Clin. Nutr., 89 (2009) 1997S-2008S. [10] G.D. Carter, J.L. Berry, E. Gunter, G. Jones, J.C. Jones, H.L. Makin, S. Sufi, M.J. Wheeler, J. Steroid Biochem. Mol. Biol., 121 (2010) 176-179. [11] K. Sarafin, N. Hidiroglou, S.P. Brooks, Open Clin Chem J, 4 (2011) 45-49. [12] Vitamin D External Quality Assessment Scheme (DEQAS) 25-OHD report, July 2009 distribution. London, UK: Charing Cross Hospital. [13] W. Jian, R.W. Edom, N. Weng, Bioanalysis, 4 (2012) 2431-2434. [14] R.J. Singh, R.L. Taylor, G.S. Reddy, S.K. Grebe, J. Clin. Endocrinol. Metab., 91 (2006) 30553061. [15] B.W. Hollis, Am. J. Clin. Nutr., 88 (2008) 507S-510S. [16] G.D. Carter, Clin. Chem., 55 (2009) 1300-1302. [17] H.J. Roth, H. Schmidt-Gayk, H. Weber, C. Niederau, Ann. Clin. Biochem., 45 (2008) 153-159. [18] J.E. Zerwekh, Am. J. Clin. Nutr., 87 (2008) 1087S-1091S. [19] A.C. Heijboer, M.A. Blankenstein, I.P. Kema, M.M. Buijs, Clin. Chem., 58 (2012) 543-548. [20] R.L. Horst, J. Steroid Biochem. Mol. Biol., 121 (2010) 180-182.
28
Page 28 of 38
*Highlights (for review)
Highlights
An LC-MS/MS based method for 25(OH)D2 and 25(OH)D3 quantification
ip t
was validated. A complete stability profile of standard solutions and QC samples was
cr
obtained.
No differences were found between matching serum, heparin, and EDTA
25(OH)D concentrations measured were close to the DEQAS LC-MS/MS
an
us
plasma samples.
Method Means.
The LC-MS/MS method showed limited correlation to DiaSorin Liaison
M
Ac ce p
te
d
25(OH)D assay.
Page 29 of 38
Figure 1
Ac ce p
te
d
M
an
us
cr
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Figure 1. Structures and metabolism of vitamin D.
Page 30 of 38
Figure 2
Ac ce p
te
d
M
an
us
cr
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Figure 2. Typical mass chromatograms of an extracted 50 ng/mL QC sample (A) and a normal human serum sample (B). MRM transitions: 25(OH)D2, m/z 413.3→355.2; [2H]3-25(OH)D2, m/z 416.3→358.2; 25(OH)D3, m/z 401.3→365.2; and [2H]3-25(OH)D3, m/z 404.3→368.
Page 31 of 38
Figure 3
Ac ce p
te
d
M
an
us
cr
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Figure 3. Comparison with the LC-MS/MS Method Means of total 25(OH)D in DEQAS samples. Regression analysis was performed on total 25(OH)D concentrations measured by our current method and the LC-MS/MS Method Mean values provided by DEQAS.
Page 32 of 38
cr
ip t
Figure 4
us
Figure 4. Comparison with DiaSorin Liaison 25(OH)D TOTAL assay. Regression analysis was performed on total 25(OH)D concentrations measured by our LC-MS/MS method and the DiaSorin Liaison 25(OH)D Total Assay in (a) 145 human serum samples, and (b) 128 of the 145 human serum samples that had DiaSorin assay measurements lower than 60 ng/mL. (b)
Ac
ce pt
ed
M an
(a)
Page 33 of 38
Table 1
Table 1. Intra-and inter-assay accuracy and precision Intra-assay
-6.09 3.77 7.42 0.24 -4.97 4.33
2.06 4.26 2.42 2.16 3.24 2.64
Mean Calculated Concentration (ng/mL) 7.20 52.0 85.6 7.74 47.0 81.8
ip t
%CV (n=6)
Accuracy (% Bias)
%CV (n=18)
-4.02 4.06 7.05 3.24 -6.08 2.21
5.03 3.63 3.04 4.42 3.37 3.63
Ac ce p
te
d
M
an
25(OH)D3
7.50 50.0 80.0 7.50 50.0 80.0
Accuracy (% Bias)
cr
25(OH)D2
Mean Calculated Concentration (ng/mL) 7.04 51.9 85.9 7.52 47.5 83.5
Inter-assay
us
Analyte
Norminal Concentration (ng/mL)
Page 34 of 38
cr
ip t
Table 2
Table 2. Sample matrix comparison
Heparin Plasma (ng/mL)
1
No Peak
No Peak
2
No Peak
3
% Difference of Heparin Plasma from Serum
EDTA Plasma (ng/mL)
% Difference of EDTA Plasma from Serum
25(OH)D3
Serum (ng/mL)
Heparin Plasma (ng/mL)
% Difference of Heparin Plasma from Serum
EDTA Plasma (ng/mL)
% Difference of EDTA Plasma from Serum
No Peak
34.9
36.3
4.01
32.4
-7.16
No Peak
No Peak
22.0
26.2
19.1
24.0
9.09
No Peak
No Peak
No Peak
41.3
41.7
0.97
38.5
-6.78
4
39.2
39.7
7.83
7.85
0.26
7.66
-2.17
5
No Peak
No Peak
47.8
53.4
11.7
48.0
0.42
6
33.6
36.4
11.7
13.2
12.8
12.1
3.42
7
No Peak
No Peak
No Peak
38.7
41.1
6.20
39.5
2.07
8
No Peak
No Peak
No Peak
32.1
37.9
18.1
32.1
0.00
9
No Peak
No Peak
No Peak
57.3
59.7
4.19
60.4
5.41
10
No Peak
No Peak
No Peak
30.8
30.8
0.00
28.2
-8.44
11
No Peak
No Peak
No Peak
35.3
36.4
3.12
33.3
-5.67
12
No Peak
No Peak
No Peak
44.0
43.8
-0.45
43.0
-2.27
13
No Peak
No Peak
No Peak
30.1
30.2
0.33
28.0
-6.98
14
No Peak
No Peak
No Peak
12.1
10.3
-14.9
9.48
-21.7
15
No Peak
No Peak
No Peak
36.1
35.7
-1.11
35.1
-2.77
16
No Peak
No Peak
No Peak
27.5
27.3
-0.73
26.4
-4.00
M an
Serum (ng/mL)
1.28
39.9
8.33
ed
Sample ID
us
25(OH)D2
1.79
No Peak
Ac
ce pt
32.9
-2.08
Page 35 of 38
ip t No Peak
18
5.43
6.04
19
No Peak
No Peak
No Peak
20
No Peak
No Peak
No Peak
21
No Peak
No Peak
22
No Peak
23
cr
No Peak
No Peak
17.2
17.9
4.07
16.8
-2.33
17.4
17.3
-0.57
15.2
-12.6
18.2
20.8
14.3
18.2
0.00
38.7
37.0
-4.39
36.3
-6.20
No Peak
36.6
39.5
7.92
33.6
-8.20
No Peak
No Peak
31.6
29.1
-7.91
27.5
-13.0
No Peak
No Peak
No Peak
18.6
17.5
-5.91
18.2
-2.15
24
No Peak
No Peak
No Peak
22.9
22.4
-2.18
22.4
-2.18
25
No Peak
No Peak
No Peak
21.4
24.7
15.4
21.0
-1.87
5.09
-6.26
Mean
6.95 0
-2.19
3.37
-3.84
0
3
1
Ac
ce pt
% Difference > 15%
ed
M an
11.2
us
17
Page 36 of 38
Table 3
Table 3. Stock solution and QC sample stability 25(OH)D3
-4.25% difference from freezer storage
-4.72% difference from freezer storage
Freezer Stability 181 day
14.7% difference from day 1
8.64% difference from day 1
QC Sample Stability
Mean (% bias from nominal concentration) Low QC Medium QC High QC (7.50 ng/mL) (50.0 ng/mL) (80.0 ng/mL) 7.21 83.9 ND (-3.89%) (4.90%)
Mean (% bias from nominal concentration) Low QC Medium QC High QC (7.50 ng/mL) (50.0 ng/mL) (80.0 ng/mL) 7.69 78.5 ND (2.58%) (-1.88%)
Bench-top 24 hour
7.44 (-0.80%)
ND
85.4 (6.79%)
Re-injection Reproducibility (% change from original injection)
-3.88%
3.53%
2.87%
Processed Sample/Autosampler Stability
7.00 (-6.73%)
51.7 (3.47%)
Freezer Stability 265 day
7.82 (4.27%)
49.2 (-1.70%)
7.98 (6.40%)
ND
87.9 (9.91%)
1.11%
1.19%
0.48%
85.4 (6.69%)
7.61 (1.47%)
48.1 (-3.83%)
83.2 (4.04%)
84.9 (6.17%)
7.55 (0.64%)
47.7 (-4.60%)
74.9 (-6.42%)
Ac ce p
te
d
M
an
us
Freeze/thaw Stability 4x
ip t
25(OH)D2
cr
Stock Solution Stability Room Temperature 6 hour
Page 37 of 38
Table 4
Measured Concentration (ng/mL)
% Difference From DEQAS Mean
386
12.2
12.2
0.00
387
24.1
24.9
3.32
388
33.1
32.6
-1.51
389
19.4
19.8
2.06
390
28.5
31.2
9.47
391
6.70
5.57
392
29.5
29.3
393
12.0
11.5
394
23.6
24.5
395
16.1
17.1
an -16.9 -0.68
d
M
-4.17 3.81 6.21 0.17
Ac ce p
te
Average
cr
DEQAS LC-MS/MS Method Mean (ng/mL)
us
DEQAS Sample ID
ip t
Table 4. Comparison with LC/MS/MS Method Means of total 25(OH)D in DEQAS Samples
Page 38 of 38
S1570-0232(14)00306-7 http://dx.doi.org/doi:10.1016/j.jchromb.2014.05.006 CHROMB 18929
To appear in:
Journal of Chromatography B
Received date: Revised date: Accepted date:
28-2-2014 29-4-2014 4-5-2014
Please cite this article as: S.W. Zhang, W. Jian, S. Sullivan, B. Sankaran, R.W. Edom, N. Weng, D. Sharkey, Development and Validation of an LCMS/MS Based Method for Quantification of 25 Hydroxyvitamin D2 and 25 Hydroxyvitamin D3 in Human Serum and Plasma, Journal of Chromatography B (2014), http://dx.doi.org/10.1016/j.jchromb.2014.05.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Development and Validation of an LC-MS/MS Based Method for Quantification of 25 Hydroxyvitamin D2 and 25 Hydroxyvitamin D3 in Human Serum and Plasma
cr
Richard W. Edomb, Naidong Wengb, and David Sharkeya
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Stanley (Weihua) Zhanga*, Wenying Jianb, Sheryl Sullivana, Banu Sankarana,
a
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Ortho Clinical Diagnostics, Johnson & Johnson, 1001 Route 202 North, Raritan,
New Jersey 08869, United States b
an
Janssen Research and Development, Johnson & Johnson, 1400 McKean Road,
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Spring House, Pennsylvania 19477, United States
* Corresponding author at: Ortho Clinical Diagnostics, Johnson & Johnson, 1001
d
Route 202 North, Raritan, New Jersey 08869, United States. Tel.: +1 908 218
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8735. E-mail address: [email protected] (Stanley Weihua Zhang).
1
Page 1 of 38
Abstract Vitamin D deficiency is increasing in the general population and has become a serious public health risk globally. As a reliable clinical indicator of
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vitamin status, 25 hydroxyvitamin D (25(OH)D) has been measured by various
cr
methods. However, the accuracy of these measurements has been the subject of considerable debate. Here, we report the development and validation of a liquid
us
chromatography-triple quadrupole mass spectrometry based method for the
an
quantification of 25(OH)D2 and 25(OH)D3 in human serum and plasma samples. Samples were first processed by protein precipitation to release the analytes
M
from the vitamin D binding protein (DBP), followed by a liquid-liquid extraction procedure. Analysis was performed on an LC-MS/MS system which utilized an
d
AB Sciex API 3000 mass spectrometer. A six point calibration curve ranging from
te
2.5 - 100 ng/mL was established for both 25(OH)D2 and 25(OH)D3. A complete
Ac ce p
method validation was conducted, including intra- and inter-assay accuracy and precision, LLOQ, dilution QC, specificity, recovery, matrix effect, and a thorough stability profile of stock solutions and QC samples. Matching samples of serum and plasma (containing either heparin or EDTA anticoagulant) generated from the same blood samples were tested, and no significant differences in 25(OH)D2 and 25(OH)D3 concentrations were found in these sample matrices. In method comparison, we analyzed 10 serum samples obtained from the Vitamin D External Quality Assessment Scheme (DEQAS), and the total 25(OH)D concentrations measured by our method were very close to the LC-MS/MS Method Mean values provided by DEQAS (average 0.17% bias, R² = 0.99). 2
Page 2 of 38
However, comparison with the DiaSorin Liaison 25(OH)D TOTAL Assay demonstrated limited correlation between these two methods (R² = 0.54). In general, concentrations measured by our LC-MS/MS method were roughly 9%
ip t
higher than those measured by the DiaSorin Liaison assay. The correlation with
DiaSorin Liaison measurement was better for samples in the lower concentration
cr
range. In summary, we developed and validated an LC-MS/MS based method
us
that can be reliably applied in routine quantification of 25(OH)D2 and 25(OH)D3 in human serum and plasma samples. This method is not suitable for pediatric
an
determinations due to the potential interference of 3-epi 25(OH)D3.
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Keywords
Abbreviations
te
comparison, stability.
d
25-hydroxyvitamin D, LC-MS/MS, validation, matrix comparison, method
Ac ce p
25(OH)D, 25-hydroxyvitamin D; 25(OH)D2, 25-hydroxyvitamin D2;
25(OH)D3, 25-hydroxyvitamin D3; 1,25(OH)2D, 1,25-dihydroxyvitamin D; vitamin D binding protein (DBP); RIA, radioimmunoassay; ELISA, enzyme-linked immunosorbent assay; HPLC, high performance liquid chromatography; LCMS/MS, liquid chromatography-tandem mass spectrometry; DEQAS, the Vitamin D External Quality Assessment Scheme; QC, Quality control; MRM, Multiple reaction monitoring; BSA, Bovine serum albumin; PBS, phosphate-buffered saline; CV, coefficients of variation; LLOQ, lower limit of quantification; EDTA, ethylenediaminetetraacetic acid; CLIA, chemiluminescence immunoassay. 3
Page 3 of 38
1. Introduction Vitamin D, including vitamin D2 and vitamin D3, is essential for the bone health of humans. Clinical trials have shown that oral vitamin D supplements
ip t
helped to prevent fractures [1]. Vitamin D deficiency has also been implicated in
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various cancers [2], cardiovascular diseases [3], and autoimmune diseases [4].
Vitamin D deficiency is increasing in the general population and is considered an
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important public health problem. The Institute of Medicine (IOM) released new
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dietary reference intakes for calcium and vitamin D in 2011, defining four categories of vitamin D status based on serum 25-hydroxyvitamin D (25(OH)D):
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(i) risk of deficiency, (ii) risk of inadequacy, (iii) sufficiency, and (iv) above which there may be reason for concern [5]. Based on these four categories, the US
d
Centers for Disease Control and Prevention (CDC) reported that 24% of the US
te
population of one year and older were at risk of inadequacy, and 8% were at risk
Ac ce p
of deficiency in 2001-2006 [6]. Experts in the field have argued that higher concentration levels should be set to define inadequacy and deficiency, thus making a higher percentage of the population fall into these categories [7]. In humans, vitamin D3 is produced from its precursor 7-
dehydrocholesterol during exposure to ultraviolet rays in sunlight, or it can be consumed in the diet. The human body does not make vitamin D2, and the normally low level of vitamin D2 in humans is from dietary intake. Vitamin D is biologically inactive, and it requires enzymatic conversion to produce active metabolites. As shown in Figure 1, Vitamin D is converted to 25(OH)D, the major circulating form of vitamin D, and then to 1,25-dihydroxyvitamin D (1,25(OH)2D), 4
Page 4 of 38
the active form of vitamin D, by enzymes in the liver and kidney [8]. Although 1,25(OH)2D is the active metabolite of vitamin D, serum levels of 1,25(OH)2D do not reflect the body’s storage and are not useful for determining vitamin D status.
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Instead, 25(OH)D concentration is accepted as a reliable clinical indicator of vitamin D status [9].
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Traditionally, 25(OH)D has been measured by radioimmunoassay (RIA),
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enzyme-linked immunosorbent assay (ELISA) and high performance liquid chromatography (HPLC). In recent years, liquid chromatography-tandem mass
an
spectrometry (LC-MS/MS) has become a popular method based on its superior specificity and ability to measure 25(OH)D2 and 25(OH)D3 separately but
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simultaneously. The method validation in publications based on LC-MS/MS
d
techniques has mostly focused on key elements of the assays, such as linearity,
te
accuracy and precision, specificity, and recovery, etc., but missed other important items such as stabilities and matrix effects. Here, we report a complete
Ac ce p
validation of an LC-MS/MS based method for the quantification of 25(OH)D2 and 25(OH)D3 in human serum and plasma. To address the issue of lack of human serum or plasma samples that had no endogenous 25(OH)D3, we used 5% bovine serum albumin (BSA) as a surrogate matrix for calibration curves as well as lower limit of quantification (LLOQ) quality control (QC) samples, and a horse serum pool, which contained marginal levels of vitamin D as a surrogate matrix for low level QC (LQC) samples. Our validation included detailed experiments on stability of test samples, stock solutions, and processed samples, as well as comparisons between matching serum, heparin, and EDTA plasma samples. 5
Page 5 of 38
This method was compared with other LC-MS/MS methods based on measurement of 10 samples from the Vitamin D External Quality Assessment Scheme (DEQAS, http://deqas.kpmd.co.uk) [10], and to the widely used DiaSorin
applications (http://www.diasorin.com/en/node/8945/) [11].
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2. Material and Methods
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Liaison 25(OH)D TOTAL Assay, an immunoassay approved by FDA for clinical
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2.1. Chemicals and Reagents
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The vitamin D related chemicals were purchased from the following sources: Vitamin D2 and vitamin D3, Sigma-Aldrich (St. Louis, MO); 25(OH)D2,
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BePharm Ltd. (Shanghai, China); 25(OH)D3 and 1,25(OH)2D3, Toronto Research Chemicals, Inc. (Toronto, Ontario, Canada); 1,25(OH)2D2 and
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24,25(OH)2D3, Santa Cruz Biotechnology (Dallas, Texas). 3-epi 25(OH)D3,
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internal standards [2H]3-25(OH)D2 and [2H]3-25(OH)D3, IsoSciences (King of
Ac ce p
Prussia, PA). Other chemicals and reagents were purchased from the following sources: HPLC grade methanol and acetonitrile, EMD Chemicals (Philadelphia, PA); heptane, Mallinckrodt Chemicals (Hazelwood, MO); ammonium acetate, Fluka (St. Louis, MO); Dulbecco's phosphate-buffered saline (PBS), Mediatech, Inc. (Manassas, VA); bovine serum albumin (BSA), Sigma-Aldrich (St. Louis, MO).
2.2. Serum and Plasma samples Normal human serum samples and horse serum samples were purchased from Bioreclamation, Inc. (Hicksville, NY) and were used for making quality 6
Page 6 of 38
control (QC) samples. For matrix comparison, matching serum, heparin, and K2EDTA plasma samples from 25 healthy donors were prepared from fresh blood samples collected in a donor room at Ortho Clinical Diagnostics. For method
ip t
comparison to the DiaSorin Liaison 25(OH)D TOTAL Assay, human serum
samples were either collected in a donor room at Ortho Clinical Diagnostics or
cr
purchased from the following suppliers: Vitrologic (Charleston, SC),
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Bioreclamation, Inc. (Hicksville, NY), Complex Antibodies, Inc. (Margate, FL), and New York Biologics, Inc. (Southampton, New York). Ten DEQAS serum
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samples were obtained from the DEQAS organization.
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2.3. Sample Preparation
Thermally equilibrate calibration standard samples, QCs, study samples,
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blank samples (horse serum), and solutions at room temperature. Mix 200 µL of
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each sample with 25 µL of internal standards at 320 ng/mL in methanol for both
Ac ce p
[2H]3-25(OH)D2 and [2H]3-25(OH)D2 in a 13x100 mm glass culture tube and equilibrate for 5 min. Add 200 µL of methanol to each tube, vortex for 10 seconds, and equilibrate for 5 minutes. Add 2 mL of heptane to each tube and vortex the tubes on a Multi-Tube Vortexer at 1600 rpm for 10 minutes. Centrifuge the tubes at 3220 rcf for 5 minutes. Transfer 1.6 mL of the top organic layer to a 2-mL 96well plate and evaporate to dryness under nitrogen at room temperature. Add 100 µL of reconstitution solution (2 mM ammonium acetate, 0.1% formic acid in 80/20: methanol/water (v/v)) to each well. Vortex the plate on a plate shaker at 1000 rpm for 2 minutes, and subject the samples to LC/MS/MS analysis. 7
Page 7 of 38
2.4. LC-MS/MS LC-MS/MS was performed on a Shimadzu HPLC system, which consisted of Shimadzu LC10AD pumps and a SIL-HTC autosampler (Kyoto, Japan),
ip t
coupled to an AB Sciex API 3000 triple-quadrupole mass spectrometer
cr
(Framingham, MA). Ten μL of each sample was injected on a Zorbax SB-C18,
3.5 µm, 2.1x50 mm HPLC column (Agilent, Santa Clara, CA). The mobile phases
us
were: (A) 2 mM ammonium acetate, 0.1% formic acid in water, and (B) 2 mM
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ammonium acetate, 0.1% formic acid in methanol. The needle rinse solvent was methanol. The HPLC flow rate was set at 0.3 mL/min. The gradient elution
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started at 80% mobile phase B for the first 0.1 min, was ramped linearly to 95% B in 0.9 min, held at 95% B for 4 min, and then returned to 80% B in 0.1 min. The
d
column was re-equilibrated with 80% B for 2.9 min prior to the next injection. The
te
LC flow was diverted to waste before 0.1 min and after 5 min of each run. The
Ac ce p
API 3000 was set to positive ionization mode, with curtain gas set at 10 psi, nebulizer gas at 10 psi, IonSpray™ voltage at 3000 V, ion source temperature at 450C and CAD gas at a nominal setting of 6. The MRM transitions for the analytes and their internal standards were: 25(OH)D2, m/z 413.3→355.2; [2H]325(OH)D2, m/z 416.3→358.2; 25(OH)D3, m/z 401.3→365.2; and [2H]3-25(OH)D3, m/z 404.3→368.2. For 25(OH)D2 and [2H]3-25(OH)D2, the declustering potential (DP), focusing potential (FP), and collision energy (CE) were set at 35 V, 140 V, and 35 eV, respectively. For 25(OH)D3 and [2H]3-25(OH)D3, the DP was set at 30 V and the FP was set at 120 V. The CE was set at 32 eV for 25(OH)D3 and
8
Page 8 of 38
30 eV for [2H]3-25(OH)D3. For all four channels, the entrance potential (EP) was set at 10 V and the collision cell exit potential (CXP) was set at 20 V. 2.5. Method Validation
ip t
Co-mingled calibration standards were prepared at levels of 2.5/2.5,
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5.0/5.0, 10/10, 25/25, 50/50, and 100/100 ng/mL (25(OH)D2 / 25(OH)D3) in 5% BSA in PBS. A co-mingled internal standard working solution was prepared in
us
methanol containing [2H]3-25(OH)D2 and [2H]3-25(OH)D3, each at a
an
concentration of 320 ng/mL. All stock solutions were prepared and stored in amber vials for protection from the light. Calibration curves were constructed by
M
plotting the chromatographic peak area ratios of 25(OH)D2 and 25(OH)D3 to their internal standards versus the corresponding concentrations, and fitting the
d
data using linear regression with a 1/x2 weighting factor. QC samples at five
te
levels (LLOQ, LQC, MQC, HQC, and dilution QC) were prepared in either 5%
Ac ce p
BSA, a pooled horse serum sample, or a pooled human serum sample. The BSA lot we used had previously been shown to devoid any 25(OH)D contamination. To determine the endogenous levels of 25(OH)D2 and 25(OH)D3 in the pooled horse serum sample and the pooled human serum sample, six replicate samples from each pool were analyzed using a calibration curve and the resulting concentration readings were averaged for each pool. Additional reference standards were then spiked in the pools to reach the desired QC concentrations. LLOQ QC was prepared in 5% BSA. LQC was prepared in the pooled horse serum, which contains minimal endogenous 25(OH)D. MQC, HQC, and dilution QC were prepared in the pooled human serum. 9
Page 9 of 38
2.5.1. Intra-assay Precision and Accuracy Six replicates each of three QC levels (low, medium, and high) were analyzed in a single run to assess intra-assay precision. The means, standard
ip t
deviations, coefficients of variation (%CV) and mean percent deviation of the
cr
back-calculated values (% Bias) were calculated at each QC concentration.
us
2.5.2. Inter-assay Precision and Accuracy
Three calibration curves were run over three analysis days. Each
an
calibration curve included six QCs at three levels (low, medium, and high). The calibration curves were each regressed separately. For each calibration curve,
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the slope, intercept, coefficient of determination, and individual percent deviations were calculated. The mean, standard deviation, %CV, and mean
d
percent deviation of the back-calculated values (% Bias) were calculated at each
Ac ce p
2.5.3. LLOQ
te
standard and QC concentration.
The accuracy and precision of the LLOQ was assessed through the
analysis of replicate QCs at the LLOQ concentration during the Inter-Assay Precision and Accuracy experiment. 2.5.4. Dilution QC
Six replicates of dilution QC samples were diluted to 50 ng/mL with 5% BSA and assayed as per the method procedure. The mean % deviation from nominal value and %CV were calculated. 10
Page 10 of 38
2.5.5. Specificity Six serum samples from a same lot were spiked with one of the six different 25(OH)D analogs (vitamin D2, vitamin D3, 1,25(OH)2D2, 1,25(OH)2D3,
ip t
24,25(OH)2D3, and 3-epi-25(OH)D3) at 200 ng/mL. The concentrations of
cr
25(OH)D2 and 25(OH)D3 before and after spiking were measured in singlet and
us
compared for detection of potential interference. 2.5.6. Recovery
an
Recovery of 25(OH)D2 during the sample preparation process was determined at three QC concentration levels (low, medium, and high). The
M
resulting chromatographic peak areas from extracted samples were compared
d
against those post-spiked to an extracted normal human serum pool which
te
contained an insignificant level of pre-existing 25(OH)D2. Internal standards were added after extraction to offset any variations from LC-MS/MS performance
Ac ce p
for both pre- and post-spiked samples. Six replicates at each concentration were assayed. The mean % recovery and %CV were calculated at each concentration. Recovery of 25(OH)D3 was determined exactly as that of 25(OH)D2
except that two QC concentration levels were used (medium and high) due to the lack of human serum samples with very low concentrations of 25(OH)D3. To account for the endogenous 25(OH)D3 concentration in the normal human serum pool, the following equation was solved to obtain the recovery value X: (Endogenous concentration + Concentration of spike)*X%
Peak Area ratio of pre-spike/internal standard =
11
Page 11 of 38
Endogenous concentration *X% + Concentration of spike
Peak Area of post-spike/internal standard
ip t
2.5.7. Matrix Effect Absolute matrix effect was determined by comparing instrument signals of
cr
analytes post-spiked in extracted biological matrix to those in neat solvent at the
us
same level. At first, six different lots of human serum samples were fully
processed in triplicate and analyzed to obtain the average concentration of
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endogenous 25(OH)D2 and 25(OH)D3 in each sample. Then, the same six lots of human serum samples (without spiking of internal standard) were extracted in
M
duplicate, evaporated to dryness, and reconstituted with analytes to reach either the medium (50 ng/mL) or the high level (80 ng/mL) QC concentration along with
d
internal standards. The analytes needed for the reconstitution was equal to the
te
desired total analytes in a reconstituted sample minus the endogenous analytes
Ac ce p
times the recovery rate determined in the “Recovery” experiment. The measurement of these reconstituted samples provided instrument signals of a known amount of 25(OH)D2 and 25(OH)D3 in the presence of human serum matrix. Finally, an identical medium or high level of the 25(OH)D2 and 25(OH)D3 was added to the reconstitution solution (no matrix) in six replicate samples. These samples were analyzed exactly as the above serum samples to obtain the mean response of the analytes in the absence of matrix. The absolute matrix factor was calculated for the analytes at each concentration as the ratio of peak response in presence of matrix ions to mean peak response in absence of matrix ions. 12
Page 12 of 38
Relative matrix effects were calculated as ratios of peak intensities of analytes over corresponding internal standards in the presence of human serum
ip t
matrix. 2.5.8. Matrix Comparison
cr
Matching serum and plasma samples (containing either heparin or K2-
EDTA anticoagulant) generated from the same blood samples from 25 healthy
us
donors were prepared from fresh blood collected in a donor room at Ortho
an
Clinical Diagnostics. 25(OH)D2 and 25(OH)D3 concentrations from these three different sample matrices from each individual were determined and compared.
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2.5.9. Stock Solution Stability
d
2.5.9.1. Room Temperature Stability
te
Room temperature stability was evaluated by comparing the instrument response of a portion of the stock solution stored at room temperature for six
Ac ce p
hours against the remainder of the solution stored in a -20°C freezer. Six replicate injections at a single concentration (10 ng/mL) for both 25(OH)D2 and 25(OH)D3 were made for each solution. 2.5.9.2. Freezer Stability
Frozen stock solutions were stored in a -20°C freezer for six months. A
solution of 25(OH)D2 and 25(OH)D3 at 10 ng/mL each was prepared from the stock solutions. Six replicate injections of the solution were made and the peak intensities were compared against those obtained from a freshly prepared stock solution from a new weighing. 13
Page 13 of 38
2.5.10. QC Sample Stability 2.5.10.1. Bench-top Stability Six replicates of QC samples at two concentration levels (low and high)
ip t
were allowed to thaw and remain at room temperature for 24 hours prior to
cr
preparation and analysis. Mean concentrations were calculated and compared
us
against the nominal concentrations. 2.5.10.2. Freezer Stability
an
Freezer stability of the study samples was evaluated by analyzing six replicates of QC samples at three concentration levels (low, medium, and high)
M
that were stored in a -20°C freezer for 265 days. A freshly prepared calibration
te
concentrations.
d
curve was used. Calculated concentrations were compared to nominal target
Ac ce p
2.5.10.3. Freeze/thaw Stability
The effect of freeze/thaw stability was tested for four cycles using six
replicates of QC samples at two concentration levels (low and high). For the first freeze/thaw cycle, the QC samples were stored at -20°C for 24 hours before being allowed to thaw at room temperature. For the second, third, and fourth cycles, the QC samples were stored at -20°C for a minimum of 12 hours before being allowed to thaw. The samples were out of the freezer for at least 1 hour between each cycle. The mean concentrations following the fourth cycle were determined for each QC concentration.
14
Page 14 of 38
2.5.11. Processed Sample/Autosampler Stability and Re-injection Reproducibility To evaluate processed sample/autosampler stability, and re-injection
ip t
reproducibility, a complete calibration curve with six replicates of QC samples at three concentration levels (low, medium, and high) were prepared and injected.
cr
At the completion of the analysis, the samples were stored in the autosampler.
us
To determine re-injection reproducibility, the calibration curve and QC samples were re-injected at 12 hours following the completion of the original run. The
an
processed sample/autosampler stability was determined after the samples had been stored in the autosampler for 72 hours. A fresh calibration curve was
M
prepared, and the original QC samples were re-injected with the freshly prepared
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2.6. Method Comparison
d
calibration curve.
Ac ce p
2.6.1. Comparison with LC-MS/MS Method Mean of Total 25(OH)D in DEQAS Samples
Ten blind serum samples from the Vitamin D External Quality Assessment
Scheme (DEQAS) were analyzed. The measured 25(OH)D2 and 25(OH)D3 concentrations were added together as total 25(OH)D concentration for each sample and compared with the corresponding LC-MS/MS Method Mean total value provided by DEQAS. 2.6.2. Comparison with DiaSorin Liaison 25(OH)D TOTAL Assay
15
Page 15 of 38
A total of 145 normal human serum samples of male and female, age 10 and older, and representing different combinations of low, medium, and high concentrations of 25(OH)D2 and 25(OH)D3 were tested in singlet. The same set
ip t
of samples was also analyzed by the chemiluminescence immunoassay (CLIA) based DiaSorin Liaison 25(OH)D TOTAL Assay at ACM Medical Laboratories
cr
(Rochester, NY). The total 25(OH)D concentrations measured by both methods
us
were compared using linear regression analysis.
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3. Results and Discussion 3.1. Method Development
M
We originally tested protein precipitation as the sample preparation method, but we encountered strong matrix effects and decreased sensitivity with
d
the API 3000 mass spectrometer. Therefore, we modified this method by adding
te
a liquid-liquid extraction step to clean up the samples following protein
Ac ce p
precipitation, which was still necessary for releasing 25(OH)D from the vitamin D binding protein (DBP). We also observed that measured 25(OH)D2 and 25(OH)D3 concentrations were higher when internal standards were added before protein precipitation (followed by a five minute equilibration) than when they were directly added in the protein precipitation solvent. We speculate that equilibration of the internal standards in the samples allowed them to bind to vitamin D binding protein, and therefore, track the recovery of the analytes more accurately. Sample preparation details such as this are very important, and different ways of handling them may contribute to the reported variability of LCMS/MS assays [12]. Figure 2 (A) and (B) show typical mass chromatograms from 16
Page 16 of 38
an extracted medium concentration level (50 ng/mL) QC sample for 25(OH)D2 and 25(OH)D3, and a normal human serum sample, respectively.
ip t
3.2. Quantification Range, Linearity, Accuracy, and Precision It is very difficult to obtain human serum or plasma samples that have no
cr
endogenous 25(OH)D3, and therefore, 5% BSA was used as a surrogate matrix
us
for calibration curves. In this case, it is important to fully evaluate the method using QC samples prepared at least at one concentration level using authentic
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matrix to demonstrate the absence of potential bias introduced by using surrogate matrix [13]. In the current method, the LLOQ QC was prepared in 5%
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BSA. A horse serum pool, which contained marginal levels of vitamin D, was screened for its baseline concentration prior to being spiked with a known
d
amount of analytes to reach the expected LQC concentration. Pooled human
te
serum was spiked on top of the measured endogenous levels to reach the
Ac ce p
designated MQC, HQC and dilution QC concentrations. Using this approach, most of the QC levels were prepared in the expected biological matrix for comprehensive evaluation of the method. Linearity was evaluated by constructing calibration curves over the
concentration range of 2.5-100 ng/mL for both 25(OH)D2 and 25(OH)D3. The r values for both analytes were ≥ 0.999. The LLOQ, defined as the lowest concentration that gave a signal-to-noise ratio of 10, was validated to be 2.5 ng/mL for both 25(OH)D2 and 25(OH)D3. To account for potentially elevated levels of 25(OH)D2 and 25(OH)D3 when vitamin D supplements or drugs were 17
Page 17 of 38
taken, a dilution QC sample at 250 ng/mL for each analyte was validated to produce acceptable accuracy and precision when it was diluted to 50 ng/mL with 5% BSA for sample analysis.
ip t
Method accuracy and precision was evaluated using QC samples at low,
cr
medium, and high concentrations. The intra-assay variability was evaluated from six replicate QC’s at three concentrations determined in a single run. The %CV
us
was 4.3 for all levels of 25(OH)D2 and 3.2 for all levels of 25(OH)D3. The
an
mean percentage deviations of the back-calculated concentrations were within 7.4% of the nominal concentrations for all levels of 25(OH)D2, and within 5.0%
M
of the nominal concentrations for all levels of 25(OH)D3. The inter-assay variability was obtained from 18 replicate QC’s at three concentrations analyzed
d
over three different days. The mean percent deviation of the back-calculated
te
concentrations were within 7.05% of the nominal concentrations for all levels of
Ac ce p
25(OH)D2 and within 6.08% of the nominal concentrations for all levels of 25(OH)D3. The %CVs were within 5.03 for all levels of 25(OH)D2, and within 4.42 for all levels of 25(OH)D3. The intra- and inter-assay accuracy and precision results were summarized in Table 1. 3.3. Specificity
To evaluate specificity of the assay, we tested six different 25(OH)D analogs (vitamin D2, vitamin D3, 1,25(OH)2D2, 1,25(OH)2D3, 24,25(OH)2D3, and 3-epi-25(OH)D3). Six serum samples from a same lot were spiked with one of the analogs at 200 ng/mL. The concentrations of 25(OH)D2 and 25(OH)D3 18
Page 18 of 38
before and after spiking were measured. If there was any interference from these analogs, the concentrations of 25(OH)D2 and/or 25(OH)D3 would have changed after spiking. Our data showed that except for 3-epi-25(OH)D3, none of the other
ip t
analogs showed any significant interference to the quantification of 25(OH)D2 and 25(OH)D3. 3-epi-25(OH)D3 shared the identical MRM transition to
cr
25(OH)D3, and it caused interference when it was not chromatographically
us
separated. However, it has been shown to be present solely in infants [14]. Although the current method could not differentiate 3-epi-25(OH)D3 from
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25(OH)D3, we did not consider further separation since this assay was not designed for an infant population.
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3.4. Recovery
d
Recovery of 25(OH)D2 during the sample preparation process was
te
determined using the low, medium, and high QC concentrations. The recovery
Ac ce p
for all concentration levels of 25(OH)D2 was ≥ 72% with a %CV ≤ 7.05. The recovery of 25(OH)D3 was only examined at the medium and high QC concentrations (Section 2.5.6). The endogenous 25(OH)D3 in the normal human serum pool used for making QC samples was accounted for during the recovery calculation. The recovery for both levels of 25(OH)D3 was ≥ 62% with a %CV ≤ 3.03. Even though the recovery for both analytes was not optimal, the acceptable performance of the QC samples indicated that the stable isotope labeled internal standards fully tracked the analytes. 3.5. Matrix Effects and Matrix Comparison 19
Page 19 of 38
Analyte signal suppression or enhancement during ionization due to biological matrix was evaluated by determining the matrix effects in six serum sample lots. Since all six serum lots had certain levels of endogenous analytes,
ip t
only the medium and high QC concentrations were examined. The serum samples were analyzed first to determine the endogenous 25(OH)D2 and
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25(OH)D3 concentrations. The endogenous 25(OH)D in each sample, multiplied
us
by the recovery rate obtained from the Recovery experiments, was the
endogenous 25(OH)D recovered during extraction. Extra 25(OH)D was spiked
an
into these extracted samples to reach the medium and high QC concentrations. The absolute matrix factor was calculated as the ratio of the peak response in
M
presence of matrix ions to the mean peak response in absence of matrix ions.
d
For 25(OH)D2, the absolute matrix factor was 0.47 (%CV 45.1) for the medium
te
level concentration and 0.13 (%CV 11.9) for the high level concentration. For 25(OH)D3, the absolute matrix factor was 0.44 (%CV 39.0) for the medium level
Ac ce p
concentration and 0.13 (%CV 18.2) for the high level concentration. Except for the high level concentration of 25(OH)D2, all results of the
absolute matrix effect experiments for 25(OH)D2 and 25(OH)D3 did not meet our acceptance criteria (%CV ≤ 15). Therefore, we evaluated relative matrix effects using peak area ratios of the analytes to their corresponding internal standards. The %CV observed for 25(OH)D2 was 1.5 for the medium level concentration and 3.5 for the high level concentration. The %CV observed for 25(OH)D3 was 1.5 for the medium level concentration and 3.0 for the high level concentration. Relative matrix effect results for both 25(OH)D2 and 25(OH)D3 thus met our 20
Page 20 of 38
acceptance criteria, indicating that the stable isotope labeled internal standards compensated for the ion suppression.
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The current assay was designed to be used with different sample matrices, such as serum and plasma. One major concern was that different
cr
anticoagulants might affect assay performance, as experienced by some LC-
MS/MS test labs [12]. To evaluate potential effects of different sample matrices
us
on assay performance, matching serum, heparin, and K2-EDTA plasma was
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generated from the same blood samples from 25 individuals. The samples were analyzed and compared, and the results were summarized in Table 2. Only 3 out
M
of 25 donors had 25(OH)D2 readings above the LLOQ (2.5 ng/mL), and the differences between heparin and EDTA plasma to serum were all smaller than
d
15%. For heparin plasma, the average difference from serum was 6.95%, and
te
for EDTA plasma it was -2.19%. For 25(OH)D3, when heparin plasma was
Ac ce p
compared to serum, 3 out of 25 donors showed a difference of >15%, but the average difference for all samples was only 3.37%. When EDTA plasma was compared to serum, only 1 out of 25 donors showed a difference of >15%, and the average difference for all samples was -3.84%. These results demonstrated that different sample matrices, serum, heparin and K2-EDTA plasma, did not cause significant variation in 25(OH)D2 and 25(OH)D3 quantification. 3.6. Stock Solution and Study Sample Stability Even though 25(OH)D2 and 25(OH)D3 in human plasma and serum has been known to be very stable, very few publications have described its complete 21
Page 21 of 38
stability profile [15]. Therefore, we conducted extensive studies to determine the stability of both analyte stock solutions and study samples, and the results were summarized in Table 3. The stock solutions for 25(OH)D2 and 25(OH)D3 were
ip t
found to be stable for 6 hours at room temperature and for 181 days in a -20°C
freezer (stability beyond 6 hours at room temperature and beyond 181 days at -
cr
20°C was not tested).
us
25(OH)D2 and 25(OH)D3 in human serum samples were found to be
an
stable at room temperature for 24 hours, and for 265 days in a -20°C freezer (stability of longer time was not tested). 25(OH)D2 and 25(OH)D3 in human
M
serum samples was stable for four freeze/thaw cycles.
The re-injection reproducibility and processed sample/autosampler
d
stability was examined with a complete calibration curve with six replicates of
te
three QC concentration levels (low, medium, and high). To determine re-injection
Ac ce p
reproducibility, the calibration curve and QC samples were re-injected following the completion of the original run. It was found that the mean concentration of each level of the QC samples was within 15% of their respective nominal value after re-injection. To determine processed sample/autosampler stability, the original QC samples were re-injected with a freshly prepared calibration curve after these samples had been stored in an autosampler at room temperature for 72 hours. 25(OH)D2 and 25(OH)D3 were both found to be stable. 3.7. Method Comparison
22
Page 22 of 38
The concentrations of total 25(OH)D in human blood measured by different testing methods can vary dramatically [16]. Since almost all human blood samples contain a certain level of 25(OH)D, there is no true blank matrix to
ip t
make QC or reference samples, and it is impossible to know if any individual
25(OH)D measurement is the true value. In order to compare the accuracy of our
cr
method to that of other LC-MS/MS methods, we took advantage of unprocessed
us
human serum samples distributed by DEQAS, which was incorporated in 1989 to ensure the analytical reliability of vitamin D assays. DEQAS samples were non-
an
spiked, normal human serum pools. DEQAS provided the LC-MS/MS Method Mean values of total 25(OH)D for each sample, which was the average test value
M
from at least one hundred different LC-MS/MS test sites [10]. Ten samples
d
(sample number 386-395) obtained from DEQAS were analyzed in singlet in a
te
blind test. The measured 25(OH)D2 and 25(OH)D3 concentrations were added together as the total 25(OH)D concentration for each sample and were compared
Ac ce p
with the corresponding LC-MS/MS Method Mean total value provided by DEQAS, as shown in Table 4 and Figure 3. The data demonstrated that our measurements were very close to DEQAS LC-MS/MS Method Mean values. The average difference from DEQAS LC-MS/MS Method Mean was 0.17%, with the largest difference at -16.87%. The regression analysis formula was LC-MS/MS = 1.0503 x DEQAS LC-MS/MS Method Mean – 0.6845, and the R2 was 0.9884, indicating excellent correlation. We also compared our method with the DiaSorin Liaison 25(OH)D TOTAL Assay, a chemiluminescence immunoassay (CLIA ) approved by FDA for 23
Page 23 of 38
measurement of total 25(OH)D concentrations in clinical applications. The DiaSorin Liaison 25(OH)D TOTAL Assay is the most popular assay in hospitals and commercial clinical laboratories. In the DEQAS July 2009 distribution report,
ip t
the DiaSorin Liaison assay was used by over 36% of about 600 laboratories [12]. Therefore, it was essential to compare the performance of our LC-MS/MS
cr
method to that of the DiaSorin Liaison assay. A total of 145 normal human serum
us
samples, representing different combinations of low, medium, and high
concentrations of 25(OH)D2 and 25(OH)D3 were tested in singlet by the LC-
an
MS/MS method, and by the DiaSorin Liaison 25(OH)D TOTAL Assay, which was performed at ACM Medical Laboratories (Rochester, NY). The total 25(OH)D
M
concentrations measured by both methods were analyzed by linear regression
d
analysis, as shown in Figure 4 (a). The resulting formula was LC-MS/MS =
te
1.0850 x DiaSorin – 3.3274, which indicated that, in general, concentrations measured by our LC-MS/MS method were roughly 9% higher than those
Ac ce p
measured by the DiaSorin Liaison assay. The R2 of 0.5439 suggested that there was limited correlation between these two methods. This result was in agreement with other published data when LC-MS/MS methods and the DiaSorin Liaison assay were compared [12, 17]. When the data were examined more closely, it appeared that there was a
better correlation between these two methods for samples of lower 25(OH)D concentrations, especially those with DiaSorin Liaison measurements lower than 60 ng/mL or so. A separate regression analysis was thus performed on all samples with measurements lower than 60 ng/mL in the DiaSorin Liaison assay 24
Page 24 of 38
(total 128 samples), and the result is shown in Figure 4 (b). The resulting formula was LC-MS/MS = 1.1953 x DiaSorin + 0.6319, and the R2 was 0.8181. Even though the LC-MS/MS readings for these samples were still roughly 20% higher
ip t
than those of the DiaSorin Liaison assay, the correlation was improved relative to that of the full sample set. Since the majority of people in the general population
cr
have total 25(OH)D concentrations less than 60 ng/mL (the proposed possibly
us
harmful level by the Institute of Medicine (IOM) was 50 ng/mL [5]), our results suggested that these two methods should correlate quite well with each other in
an
clinical tests within the general population.
M
There is no clear explanation as to why DiaSorin Liaison assays have generally provided lower 25(OH)D measurements than LC-MS/MS based
d
methods. In LC-MS/MS based methods, native analytes are normally tracked by
te
stable-isotope-labeled internal standards having identical physicochemical
Ac ce p
behaviors. Such methods should, therefore, yield the highest attainable analytical accuracy, and the LC-MS/MS based methods are widely considered the gold standard for 25(OH)D quantification[18]. One possible reason for lower DiaSorin Liaison assay readings is the potential low recovery of 25(OH)D from sample preparation. Instead of organic solvent extraction used in most LC-MS/MS based methods, the DiaSorin Liaison assay relies on pH change and/or blocking agents that liberate the 25(OH)D from the vitamin D binding protein (DBP). Any possible loss of target that occurs during the extraction method will result in lower recovery without proper internal standard for correction. Indeed, a recent report showed that an inverse relationship between the DBP concentrations and relative 25
Page 25 of 38
recovery was observed in the DiaSorin Liaison assay when it was compared to an LC-MS/MS based method [19]. In another report, low recovery was observed for exogenously added 25(OH)D in human serum samples, and it was suspected
ip t
that the exogenous 25(OH)D did not distribute properly on the DBP [20]. More
cause of low recovery in the DiaSorin Liaison assay.
us
4. Conclusion
cr
experiments are needed to examine whether the extraction method is the true
an
We have developed an LC-MS/MS based method for quantification of 25(OH)D2 and 25(OH)D3 in human serum and plasma and conducted a
M
validation covering accuracy and precision, LLOQ, dilution QC, specificity, recovery, matrix effect, and a thorough stability profile of stock solutions and QC
d
samples. The extraction method, which consisted of a combination of methanol
te
precipitation and liquid-liquid extraction by heptane, produced a relatively clean
Ac ce p
extract compatible with most triple-quadrupole instruments. Matrix comparisons between matching serum, heparin, and EDTA plasma samples demonstrated that this LC-MS/MS method was suitable for these sample matrices without producing bias among the results. This assay should not be used for pediatric determinations without further chromatographic development due to the potential interference of 3-epi 25(OH)D3. Our method was shown to have accuracy that closely followed the LCMS/MS Method Mean when 10 DEQAS samples were tested in blind. On the other hand, in a test of 145 normal human serum samples from males and 26
Page 26 of 38
females of age 10 and up, the total 25(OH)D concentrations obtained by our LCMS/MS assay were, in general, about 9% higher than those obtained by the DiaSorin Liaison 25(OH)D TOTAL Assay, and there was limited correlation (R2 =
ip t
0.5439) between these two methods. The correlation was better for samples with
cr
DiaSorin Liaison measurements lower than 60 ng/mL (R2 = 0.8181).
us
Acknowledgements
We wish to thank Ms. Susan Chazan and Dr. Lucius (Tad) Fox for their
an
inputs in method validation; Dr. Wensheng Lang for his assistance in the early
Ac ce p
te
d
M
stage of the project.
27
Page 27 of 38
References:
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te
d
M
an
us
cr
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[1] H.A. Bischoff-Ferrari, W.C. Willett, J.B. Wong, A.E. Stuck, H.B. Staehelin, E.J. Orav, A. Thoma, D.P. Kiel, J. Henschkowski, Arch. Intern. Med., 169 (2009) 551-561. [2] C. Buttigliero, C. Monagheddu, P. Petroni, A. Saini, L. Dogliotti, G. Ciccone, A. Berruti, Oncologist, 16 (2011) 1215-1227. [3] C. McGreevy, D. Williams, Ann. Intern. Med., 155 (2011) 820-826. [4] A. Ascherio, K.L. Munger, K.C. Simon, Lancet Neurol., 9 (2010) 599-612. [5] A.C. Ross, C.L. Taylor, A.L. Yaktine, H.B. Del Valle, Institute of Medicine of National Academies, The National Academies Press, Washington, DC., (2010). [6] A.C. Looker, C.L. Johnson, D.A. Lacher, C. Pfeiffer, R.L. Schleicher, C.T. Sempos, NCHS Data Brief, 59 (2011). [7] R.P. Heaney, M.F. Holick, J. Bone Miner. Res., 26 (2011) 455-457. [8] S. Christakos, D.V. Ajibade, P. Dhawan, A.J. Fechner, L.J. Mady, Endocrinol. Metab. Clin. North Am., 39 (2010) 243-253, table of contents. [9] K.M. Seamans, K.D. Cashman, Am. J. Clin. Nutr., 89 (2009) 1997S-2008S. [10] G.D. Carter, J.L. Berry, E. Gunter, G. Jones, J.C. Jones, H.L. Makin, S. Sufi, M.J. Wheeler, J. Steroid Biochem. Mol. Biol., 121 (2010) 176-179. [11] K. Sarafin, N. Hidiroglou, S.P. Brooks, Open Clin Chem J, 4 (2011) 45-49. [12] Vitamin D External Quality Assessment Scheme (DEQAS) 25-OHD report, July 2009 distribution. London, UK: Charing Cross Hospital. [13] W. Jian, R.W. Edom, N. Weng, Bioanalysis, 4 (2012) 2431-2434. [14] R.J. Singh, R.L. Taylor, G.S. Reddy, S.K. Grebe, J. Clin. Endocrinol. Metab., 91 (2006) 30553061. [15] B.W. Hollis, Am. J. Clin. Nutr., 88 (2008) 507S-510S. [16] G.D. Carter, Clin. Chem., 55 (2009) 1300-1302. [17] H.J. Roth, H. Schmidt-Gayk, H. Weber, C. Niederau, Ann. Clin. Biochem., 45 (2008) 153-159. [18] J.E. Zerwekh, Am. J. Clin. Nutr., 87 (2008) 1087S-1091S. [19] A.C. Heijboer, M.A. Blankenstein, I.P. Kema, M.M. Buijs, Clin. Chem., 58 (2012) 543-548. [20] R.L. Horst, J. Steroid Biochem. Mol. Biol., 121 (2010) 180-182.
28
Page 28 of 38
*Highlights (for review)
Highlights
An LC-MS/MS based method for 25(OH)D2 and 25(OH)D3 quantification
ip t
was validated. A complete stability profile of standard solutions and QC samples was
cr
obtained.
No differences were found between matching serum, heparin, and EDTA
25(OH)D concentrations measured were close to the DEQAS LC-MS/MS
an
us
plasma samples.
Method Means.
The LC-MS/MS method showed limited correlation to DiaSorin Liaison
M
Ac ce p
te
d
25(OH)D assay.
Page 29 of 38
Figure 1
Ac ce p
te
d
M
an
us
cr
ip t
Figure 1. Structures and metabolism of vitamin D.
Page 30 of 38
Figure 2
Ac ce p
te
d
M
an
us
cr
ip t
Figure 2. Typical mass chromatograms of an extracted 50 ng/mL QC sample (A) and a normal human serum sample (B). MRM transitions: 25(OH)D2, m/z 413.3→355.2; [2H]3-25(OH)D2, m/z 416.3→358.2; 25(OH)D3, m/z 401.3→365.2; and [2H]3-25(OH)D3, m/z 404.3→368.
Page 31 of 38
Figure 3
Ac ce p
te
d
M
an
us
cr
ip t
Figure 3. Comparison with the LC-MS/MS Method Means of total 25(OH)D in DEQAS samples. Regression analysis was performed on total 25(OH)D concentrations measured by our current method and the LC-MS/MS Method Mean values provided by DEQAS.
Page 32 of 38
cr
ip t
Figure 4
us
Figure 4. Comparison with DiaSorin Liaison 25(OH)D TOTAL assay. Regression analysis was performed on total 25(OH)D concentrations measured by our LC-MS/MS method and the DiaSorin Liaison 25(OH)D Total Assay in (a) 145 human serum samples, and (b) 128 of the 145 human serum samples that had DiaSorin assay measurements lower than 60 ng/mL. (b)
Ac
ce pt
ed
M an
(a)
Page 33 of 38
Table 1
Table 1. Intra-and inter-assay accuracy and precision Intra-assay
-6.09 3.77 7.42 0.24 -4.97 4.33
2.06 4.26 2.42 2.16 3.24 2.64
Mean Calculated Concentration (ng/mL) 7.20 52.0 85.6 7.74 47.0 81.8
ip t
%CV (n=6)
Accuracy (% Bias)
%CV (n=18)
-4.02 4.06 7.05 3.24 -6.08 2.21
5.03 3.63 3.04 4.42 3.37 3.63
Ac ce p
te
d
M
an
25(OH)D3
7.50 50.0 80.0 7.50 50.0 80.0
Accuracy (% Bias)
cr
25(OH)D2
Mean Calculated Concentration (ng/mL) 7.04 51.9 85.9 7.52 47.5 83.5
Inter-assay
us
Analyte
Norminal Concentration (ng/mL)
Page 34 of 38
cr
ip t
Table 2
Table 2. Sample matrix comparison
Heparin Plasma (ng/mL)
1
No Peak
No Peak
2
No Peak
3
% Difference of Heparin Plasma from Serum
EDTA Plasma (ng/mL)
% Difference of EDTA Plasma from Serum
25(OH)D3
Serum (ng/mL)
Heparin Plasma (ng/mL)
% Difference of Heparin Plasma from Serum
EDTA Plasma (ng/mL)
% Difference of EDTA Plasma from Serum
No Peak
34.9
36.3
4.01
32.4
-7.16
No Peak
No Peak
22.0
26.2
19.1
24.0
9.09
No Peak
No Peak
No Peak
41.3
41.7
0.97
38.5
-6.78
4
39.2
39.7
7.83
7.85
0.26
7.66
-2.17
5
No Peak
No Peak
47.8
53.4
11.7
48.0
0.42
6
33.6
36.4
11.7
13.2
12.8
12.1
3.42
7
No Peak
No Peak
No Peak
38.7
41.1
6.20
39.5
2.07
8
No Peak
No Peak
No Peak
32.1
37.9
18.1
32.1
0.00
9
No Peak
No Peak
No Peak
57.3
59.7
4.19
60.4
5.41
10
No Peak
No Peak
No Peak
30.8
30.8
0.00
28.2
-8.44
11
No Peak
No Peak
No Peak
35.3
36.4
3.12
33.3
-5.67
12
No Peak
No Peak
No Peak
44.0
43.8
-0.45
43.0
-2.27
13
No Peak
No Peak
No Peak
30.1
30.2
0.33
28.0
-6.98
14
No Peak
No Peak
No Peak
12.1
10.3
-14.9
9.48
-21.7
15
No Peak
No Peak
No Peak
36.1
35.7
-1.11
35.1
-2.77
16
No Peak
No Peak
No Peak
27.5
27.3
-0.73
26.4
-4.00
M an
Serum (ng/mL)
1.28
39.9
8.33
ed
Sample ID
us
25(OH)D2
1.79
No Peak
Ac
ce pt
32.9
-2.08
Page 35 of 38
ip t No Peak
18
5.43
6.04
19
No Peak
No Peak
No Peak
20
No Peak
No Peak
No Peak
21
No Peak
No Peak
22
No Peak
23
cr
No Peak
No Peak
17.2
17.9
4.07
16.8
-2.33
17.4
17.3
-0.57
15.2
-12.6
18.2
20.8
14.3
18.2
0.00
38.7
37.0
-4.39
36.3
-6.20
No Peak
36.6
39.5
7.92
33.6
-8.20
No Peak
No Peak
31.6
29.1
-7.91
27.5
-13.0
No Peak
No Peak
No Peak
18.6
17.5
-5.91
18.2
-2.15
24
No Peak
No Peak
No Peak
22.9
22.4
-2.18
22.4
-2.18
25
No Peak
No Peak
No Peak
21.4
24.7
15.4
21.0
-1.87
5.09
-6.26
Mean
6.95 0
-2.19
3.37
-3.84
0
3
1
Ac
ce pt
% Difference > 15%
ed
M an
11.2
us
17
Page 36 of 38
Table 3
Table 3. Stock solution and QC sample stability 25(OH)D3
-4.25% difference from freezer storage
-4.72% difference from freezer storage
Freezer Stability 181 day
14.7% difference from day 1
8.64% difference from day 1
QC Sample Stability
Mean (% bias from nominal concentration) Low QC Medium QC High QC (7.50 ng/mL) (50.0 ng/mL) (80.0 ng/mL) 7.21 83.9 ND (-3.89%) (4.90%)
Mean (% bias from nominal concentration) Low QC Medium QC High QC (7.50 ng/mL) (50.0 ng/mL) (80.0 ng/mL) 7.69 78.5 ND (2.58%) (-1.88%)
Bench-top 24 hour
7.44 (-0.80%)
ND
85.4 (6.79%)
Re-injection Reproducibility (% change from original injection)
-3.88%
3.53%
2.87%
Processed Sample/Autosampler Stability
7.00 (-6.73%)
51.7 (3.47%)
Freezer Stability 265 day
7.82 (4.27%)
49.2 (-1.70%)
7.98 (6.40%)
ND
87.9 (9.91%)
1.11%
1.19%
0.48%
85.4 (6.69%)
7.61 (1.47%)
48.1 (-3.83%)
83.2 (4.04%)
84.9 (6.17%)
7.55 (0.64%)
47.7 (-4.60%)
74.9 (-6.42%)
Ac ce p
te
d
M
an
us
Freeze/thaw Stability 4x
ip t
25(OH)D2
cr
Stock Solution Stability Room Temperature 6 hour
Page 37 of 38
Table 4
Measured Concentration (ng/mL)
% Difference From DEQAS Mean
386
12.2
12.2
0.00
387
24.1
24.9
3.32
388
33.1
32.6
-1.51
389
19.4
19.8
2.06
390
28.5
31.2
9.47
391
6.70
5.57
392
29.5
29.3
393
12.0
11.5
394
23.6
24.5
395
16.1
17.1
an -16.9 -0.68
d
M
-4.17 3.81 6.21 0.17
Ac ce p
te
Average
cr
DEQAS LC-MS/MS Method Mean (ng/mL)
us
DEQAS Sample ID
ip t
Table 4. Comparison with LC/MS/MS Method Means of total 25(OH)D in DEQAS Samples
Page 38 of 38
