Clin Chem Lab Med 2015; aop
Emre İspir*, Muhittin A. Serdar, Taner Ozgurtas, Ozlem Gulbahar, Kadir Okhan Akın, Fatih Yesildal and İsmail Kurt
Comparison of four automated serum vitamin B12 assays DOI 10.1515/cclm-2014-0843 Received August 21, 2014; accepted December 1, 2014
Abstract Background: Diagnosis of vitamin B12 deficiency is generally based on the measurement of serum vitamin B12 levels. However, in selected cases functional indices of vitamin B12, such as methylmalonic acid (MMA) and homocysteine (HCY), are needed. Here we compare the performance of four automated total vitamin B12 assays and also investigate how these assays relate to functional indices of vitamin B12 status. Methods: Total vitamin B12, MMA and HCY were measured in 69 serum samples from routine vitamin B12 assay requests. Serum vitamin B12 analysis was performed using four different immunoassay autoanalyzers: DxI 800 Unicel (Beckman Coulter, USA), ADVIA Centaur XP (Siemens Diagnostics, Tarrytown, NY, USA), Roche Cobas E601 (Roche Diagnostics, Germany), Architect i2000sr (Abbott Laboratories, Abbott Park, IL, USA). Serum MMA levels were determined by liquid chromatography-mass spectrometry (LC-MS) and serum homocysteine levels were determined by high pressure liquid chromatography (HPLC) methods. Results: Four immunoassay methods were comparable and correlated with each other. Correlation coefficients (r) ranged from 0.898 to 0.987, p < 0.001. Highest correlation was observed between Roche Cobas – Architect i2000sr and poorest correlation was observed between
*Corresponding author: Emre İspir, Laboratory Specialist, MD, Hakkari Military Hospital, Department of Medical Biochemistry, Hakkari, Turkey, Phone: +90 04382114868, E-mail: [email protected] Muhittin A. Serdar: Department of Medical Biochemistry, Acıbadem University, Istanbul, Turkey Taner Ozgurtas, Fatih Yesildal and İsmail Kurt: Department of Medical Biochemistry, Gulhane School of Medicine, Ankara, Turkey Ozlem Gulbahar: Department of Medical Biochemistry, Gazi University, School of Medicine, Ankara, Turkey Kadir Okhan Akın: Department of Medical Biochemistry, Kecioren Research and Training Hospital, Ankara, Turkey
DxI 800 Unicel – ADVIA Centaur comparison. DxI 800 Unicel assay demonstrated high mean bias [–122 pg/mL (–616–125 pg/mL)] and a concordance correlation coefficient (CCC) of 0.9161, lower than the others. MMA and HCY were correlated with the vitamin B12 results. The correlation coefficients with their 95% CI indicated that there was no statistically significant difference between the four methods according to their relationship with MMA and HCY. Conclusions: Total B12 assays correlate very well with each other. However, results of DxI 800 Unicel were lower compared to the other three autoanalyzers. All total vitamin B12 methods show similar relationships with HCY and MMA. Standardization of serum vitamin B12 assays is still not completed and further standardization studies are needed. Laboratory professionals and clinicians should be aware of this disagreement between assay methods and they should use these tests as ancillary tests. Keywords: immunoassay; method comparison; vitamin B12.
Introduction Vitamin B12 is a water-soluble vitamin that is synthesized by microorganisms and detected in trace amounts mostly in foods of animal origin. Vitamin B12 deficiency manifests itself with signs and symptoms of neurological and hematological system: Anemia, pancytopenia, macrocytosis, hypersegmented neutrophils, hypercellularity in bone marrow, ineffective erythropoiesis, paresthesia, ataxia, weakness, and impaired sense of position [1]. There is no reference method for vitamin B12 analysis yet. Diagnosis is usually made by measuring serum or plasma vitamin B12 levels [2, 3]. A level below 200 pg/mL is considered as vitamin B12 deficiency [4]. However, standardization of currently used measurement methods could not be achieved due to the lack of commutable reference materials and reference methods. This causes disagreement between vitamin B12 assays [5]. Serum vitamin B12 level alone is not sufficient to diagnose the deficiency and its sensitivity is questionable [6].
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2 İspir et al.: Comparison of four automated serum vitamin B12 assays
Therefore, measurement of methylmalonic acid (MMA) [7] and homocysteine (HCY) levels, as functional biomarkers of vitamin B12 deficiency, is recommended to evaluate the diagnosis [8]. In this study, we aimed to investigate the agreement between four fully automated cobalamin assays and how these assays relate to functional biomarkers of vitamin B12 deficiency.
Abbott and Siemens assays are standardized using cyanocobalamin (United States Pharmacopeia [USP] Reference standard). USP is mentioned considering traceability; however the precise chemical compound is not given. The traceability of the Roche assay refers to an earlier test kit of the same manufacturer. Beckman assay is standardized using cyanocobalamin and the traceability procedure is based on EN ISO 17511 (Table 1). Serum MMA concentrations were determined by liquid chromatography-mass spectrometry (LC-MS) method developed by Blom et al. [9]. Serum HCY levels were determined by high pressure liquid chromatography (HPLC) method using Immuchrom (Hessen, Germany) assay kit.
Materials and methods Statistical analyses We selected 69 serum samples from routine vitamin B12 assay requests. Inclusion criteria were normal serum folate and creatinine levels owing to the impact of renal fuction and low folate levels on MMA and HCY levels. Samples were divided into three groups which had the ranges of low vitamin B12, normal vitamin B12 and high vitamin B12 levels. Serum vitamin B12 values ranged between 85 pg/mL to 2000 pg/mL and serum folate values ranged between 5.43 ng/mL to 24 ng/mL. Serum samples were divided into multiple aliquots, and aliquots were stored at –80 °C until the day of analysis. All studies using samples from human subjects were approved by the Institutional Review Board of the Gulhane School of Medicine (Ankara, Turkey). Serum vitamin B12, serum MMA and serum HCY levels were determined for each sample. Serum vitamin B12 analysis was performed using four different immunoassay autoanalyzers and their performance characteristics are shown in Table 1: DxI 800 Unicel (Beckman Coulter, USA), ADVIA Centaur XP (Siemens Diagnostics, Tarrytown, NY, USA), Cobas E601 (Roche Diagnostics, Germany), Architect i2000sr (Abbott Laboratories, Abbott Park, IL, USA). The main principle of these methods is competetive protein binding (CPB), however they have different detection methods; electrochemiluminescence detection in Roche Cobas E601, chemiluminescence detection produced from the enzymatic reaction in DxI 800 Unicel, chemiluminescence detection in Architect i2000sr and ADVIA Centaur (Table 1).
Deming regression and concordance correlation analysis were performed according to the consensus process, by the CLSI EP9-A2: Method Comparison and Bias Estimation Using Patient Samples. Statistical analyses [paired t-test, confidence intervals (CI), correlation analysis] were carried out with SPSS 18.0 for Windows (SPSS Inc., Chicago, IL, USA). Statistical significance was accepted for p-values < 0.05. The concordance correlation coefficient (CCC) was used to evaluate the agreement between four methods.
Results A summary of serum vitamin B12, MMA and HCY concentrations in 69 samples is shown in Table 2.
Comparison of immunoassay methods and their agreement The regression analysis was performed for each assay separately. Figure 1 illustrates the graphical data of
Table 1 Summary of performance characteristics according to information provided in the vitamin B12 package inserts. Performance characteristics
DxI 800 Unicel
ADVIA Centaur XP
Roche Cobas E601
Architect i2000sr
Principle of the method
Chemiluminescent paramagnetic microparticle immunoassay Traceable to an internal standard manufactured using the purified cyanocobalamin (vitamin B12) < 50 pg/mL 50–1500 pg/mL 4.8–11.4% 126.5–505.1 pg/mL
Chemiluminescent paramagnetic microparticle immunoassay Traceable to an internal standard manufactured using U.S.P. (United States Pharmacopeia) material 45 pg/mL 45–2000 pg/mL 2.4–5% 211–911 pg/mL
Electrochemiluminescent paramagnetic microparticle immunoassay Standardized against the vitamin B12 assay
Chemiluminescent paramagnetic microparticle immunoassay Standardized using cyanocobalamin (USP Reference standard) 83 pg/mL 125–2000 pg/mL 2.7–5.6% 187–883 pg/mL
Traceability
Analytical sensitivity Linearity and precision, % Reference range
30 pg/mL 30–2000 pg/mL 1.7–4% 191–663 pg/mL
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İspir et al.: Comparison of four automated serum vitamin B12 assays 3
Table 2 Summary of serum vitamin B12, MMA and HCY concentrations in 69 samples.
Mean Median SD Concentration range
Vitamin B12, pg/mL DxI 800 Unicel
ADVIA Centaur XP
Roche Cobas E601
Architect i2000sr
347.6 211 409.5 43–2000
522.5 433 447.2 85–2000
472.5 310.4 443.7 78.6–2000
534.8 372 504.8 105–2217
regression analysis of four automated cobalamin assays. Four immunoassay methods were comparable and correlated with each other with slopes ranging from 0.798 to 1.140 and correlation coefficients (r) from 0.898 to 0.987, p < 0.001. However, in the analysis of regression equations, 95% CI for the slopes obtained for Roche Cobas – Architect i2000sr, Architect i2000sr – DxI 800 Unicel and Architect i2000sr – ADVIA Centaur comparisons did not overlap 1.0. Highest correlation was observed between Roche Cobas – Architect i2000sr and lowest correlation was observed between DxI 800 Unicel – ADVIA Centaur. In the absence of a reference method, we used the mean value of results of four automated assays as the best available surrogate for the true value in each sample. Figure 2 illustrates comparison of four methods against the mean value of vitamin B12 results. The mean values of four methods varied from 347.6 to 534.8 pg/mL (Table 2). When compared with the mean of all assays (469.3 pg/mL), the DxI 800 Unicel assay showed the greatest deviation (Figure 2). The mean bias over all samples was highest for the DxI 800 Unicel [–122 pg/mL (–616–125 pg/mL)] and lowest for the Roche Cobas [3 pg/mL (–162–183 pg/mL)]. We also evaluated the agreement between four methods using the CCC (Table 3). Pearson’s coefficients for precision, and the bias correction coefficients were ≥ 0.90 for all immunoassays. However, DxI 800 Unicel shows the lowest Pearson’s coefficients for precision and the bias correction coefficient. We attribute the reason for this discordance in DxI 800 Unicel results to the lowness of reference values used for vitamin B12. Thirty-three results from DxI 800 Unicel, 22 results from Architect i2000sr and ADVIA Centaur analyzers, 27 results from Roche Cobas analyzer were below the currently accepted deficiency level of 200 pg/mL.
Relation of the four methods with functional biomarkers MMA and HCY levels correlated inversely with vitamin B12 results of four analyzers. CLSI document C28A was
MMA, μmol/L
HCY, μmol/L
0.31 0.19 0.34 0.03–1.7
15.13 12.69 8.21 4.95–48.33
used for the determination of MMA reference range and threshold value of 0.3 μmol/L was determined. Comparison of four vitamin B12 methods with MMA is shown on Figure 3. MMA results were correlated with the vitamin B12 results. The correlation coefficients with their 95% CI were –0.337 (–0.531 to –0.109), –0.315 (–0.513 to –0.084), –0.287 (–0.490 to –0.054), –0.275 (–0.480 to –0.041) for ADVIA Centaur, Roche Cobas, Architect i2000sr, DxI 800 Unicel, respectively. We also compared the HCY levels with the vitamin B12 levels. Figure 4 depicts comparison of four vitamin B12 methods with HCY. There was a negative correlation between the vitamin B12 and HCY levels. The correlation coefficients with their 95% CI were –0.448 (–0.619 to –0.237), –0.427 (–0.603 to –0.212), –0.404 (–0.585 to –0.185), –0.341 (–0.535 to –0.113), for ADVIA Centaur, Roche Cobas, Architect i2000sr, DxI 800 Unicel, respectively. These correlation results indicated that there was no statistically significant difference between four methods according to their relationship with MMA and HCY. When the patients with low serum vitamin B12 and high HCY levels are compared to all of the patients with low serum vitamin B12 levels, the ratio was the highest for HCY and ADVIA Centaur comparison, and was the lowest for HCY and DxI 800 Unicel comparison, 16/22 (72.7%) and 17/28 (60.7%), respectively.
Discussion Today, widely used measurement methods quantitate the vitamin B12 levels in serum for the diagnosis of vitamin B12 deficiency. Vitamin B12, the largest of all vitamins, has multiple forms and very low serum levels. It binds to serum proteins tightly [10]. All these properties of vitamin B12, difficulty of producing pure reference materials and the lack of reference method make the standardization of assays more difficult [10]. The standardization of serum folate and vitamin B12 assay methods has not been achieved yet. The World
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4 İspir et al.: Comparison of four automated serum vitamin B12 assays
Slope Intercept r Paired t test
: 1.140 (1.095−1.184) : −3.721 [(−32.369)−(24.927)] : 0.9872 (0.979−0.992) :
Emre İspir*, Muhittin A. Serdar, Taner Ozgurtas, Ozlem Gulbahar, Kadir Okhan Akın, Fatih Yesildal and İsmail Kurt
Comparison of four automated serum vitamin B12 assays DOI 10.1515/cclm-2014-0843 Received August 21, 2014; accepted December 1, 2014
Abstract Background: Diagnosis of vitamin B12 deficiency is generally based on the measurement of serum vitamin B12 levels. However, in selected cases functional indices of vitamin B12, such as methylmalonic acid (MMA) and homocysteine (HCY), are needed. Here we compare the performance of four automated total vitamin B12 assays and also investigate how these assays relate to functional indices of vitamin B12 status. Methods: Total vitamin B12, MMA and HCY were measured in 69 serum samples from routine vitamin B12 assay requests. Serum vitamin B12 analysis was performed using four different immunoassay autoanalyzers: DxI 800 Unicel (Beckman Coulter, USA), ADVIA Centaur XP (Siemens Diagnostics, Tarrytown, NY, USA), Roche Cobas E601 (Roche Diagnostics, Germany), Architect i2000sr (Abbott Laboratories, Abbott Park, IL, USA). Serum MMA levels were determined by liquid chromatography-mass spectrometry (LC-MS) and serum homocysteine levels were determined by high pressure liquid chromatography (HPLC) methods. Results: Four immunoassay methods were comparable and correlated with each other. Correlation coefficients (r) ranged from 0.898 to 0.987, p < 0.001. Highest correlation was observed between Roche Cobas – Architect i2000sr and poorest correlation was observed between
*Corresponding author: Emre İspir, Laboratory Specialist, MD, Hakkari Military Hospital, Department of Medical Biochemistry, Hakkari, Turkey, Phone: +90 04382114868, E-mail: [email protected] Muhittin A. Serdar: Department of Medical Biochemistry, Acıbadem University, Istanbul, Turkey Taner Ozgurtas, Fatih Yesildal and İsmail Kurt: Department of Medical Biochemistry, Gulhane School of Medicine, Ankara, Turkey Ozlem Gulbahar: Department of Medical Biochemistry, Gazi University, School of Medicine, Ankara, Turkey Kadir Okhan Akın: Department of Medical Biochemistry, Kecioren Research and Training Hospital, Ankara, Turkey
DxI 800 Unicel – ADVIA Centaur comparison. DxI 800 Unicel assay demonstrated high mean bias [–122 pg/mL (–616–125 pg/mL)] and a concordance correlation coefficient (CCC) of 0.9161, lower than the others. MMA and HCY were correlated with the vitamin B12 results. The correlation coefficients with their 95% CI indicated that there was no statistically significant difference between the four methods according to their relationship with MMA and HCY. Conclusions: Total B12 assays correlate very well with each other. However, results of DxI 800 Unicel were lower compared to the other three autoanalyzers. All total vitamin B12 methods show similar relationships with HCY and MMA. Standardization of serum vitamin B12 assays is still not completed and further standardization studies are needed. Laboratory professionals and clinicians should be aware of this disagreement between assay methods and they should use these tests as ancillary tests. Keywords: immunoassay; method comparison; vitamin B12.
Introduction Vitamin B12 is a water-soluble vitamin that is synthesized by microorganisms and detected in trace amounts mostly in foods of animal origin. Vitamin B12 deficiency manifests itself with signs and symptoms of neurological and hematological system: Anemia, pancytopenia, macrocytosis, hypersegmented neutrophils, hypercellularity in bone marrow, ineffective erythropoiesis, paresthesia, ataxia, weakness, and impaired sense of position [1]. There is no reference method for vitamin B12 analysis yet. Diagnosis is usually made by measuring serum or plasma vitamin B12 levels [2, 3]. A level below 200 pg/mL is considered as vitamin B12 deficiency [4]. However, standardization of currently used measurement methods could not be achieved due to the lack of commutable reference materials and reference methods. This causes disagreement between vitamin B12 assays [5]. Serum vitamin B12 level alone is not sufficient to diagnose the deficiency and its sensitivity is questionable [6].
Brought to you by | University of California - San Francisco Authenticated Download Date | 2/24/15 10:29 AM
2 İspir et al.: Comparison of four automated serum vitamin B12 assays
Therefore, measurement of methylmalonic acid (MMA) [7] and homocysteine (HCY) levels, as functional biomarkers of vitamin B12 deficiency, is recommended to evaluate the diagnosis [8]. In this study, we aimed to investigate the agreement between four fully automated cobalamin assays and how these assays relate to functional biomarkers of vitamin B12 deficiency.
Abbott and Siemens assays are standardized using cyanocobalamin (United States Pharmacopeia [USP] Reference standard). USP is mentioned considering traceability; however the precise chemical compound is not given. The traceability of the Roche assay refers to an earlier test kit of the same manufacturer. Beckman assay is standardized using cyanocobalamin and the traceability procedure is based on EN ISO 17511 (Table 1). Serum MMA concentrations were determined by liquid chromatography-mass spectrometry (LC-MS) method developed by Blom et al. [9]. Serum HCY levels were determined by high pressure liquid chromatography (HPLC) method using Immuchrom (Hessen, Germany) assay kit.
Materials and methods Statistical analyses We selected 69 serum samples from routine vitamin B12 assay requests. Inclusion criteria were normal serum folate and creatinine levels owing to the impact of renal fuction and low folate levels on MMA and HCY levels. Samples were divided into three groups which had the ranges of low vitamin B12, normal vitamin B12 and high vitamin B12 levels. Serum vitamin B12 values ranged between 85 pg/mL to 2000 pg/mL and serum folate values ranged between 5.43 ng/mL to 24 ng/mL. Serum samples were divided into multiple aliquots, and aliquots were stored at –80 °C until the day of analysis. All studies using samples from human subjects were approved by the Institutional Review Board of the Gulhane School of Medicine (Ankara, Turkey). Serum vitamin B12, serum MMA and serum HCY levels were determined for each sample. Serum vitamin B12 analysis was performed using four different immunoassay autoanalyzers and their performance characteristics are shown in Table 1: DxI 800 Unicel (Beckman Coulter, USA), ADVIA Centaur XP (Siemens Diagnostics, Tarrytown, NY, USA), Cobas E601 (Roche Diagnostics, Germany), Architect i2000sr (Abbott Laboratories, Abbott Park, IL, USA). The main principle of these methods is competetive protein binding (CPB), however they have different detection methods; electrochemiluminescence detection in Roche Cobas E601, chemiluminescence detection produced from the enzymatic reaction in DxI 800 Unicel, chemiluminescence detection in Architect i2000sr and ADVIA Centaur (Table 1).
Deming regression and concordance correlation analysis were performed according to the consensus process, by the CLSI EP9-A2: Method Comparison and Bias Estimation Using Patient Samples. Statistical analyses [paired t-test, confidence intervals (CI), correlation analysis] were carried out with SPSS 18.0 for Windows (SPSS Inc., Chicago, IL, USA). Statistical significance was accepted for p-values < 0.05. The concordance correlation coefficient (CCC) was used to evaluate the agreement between four methods.
Results A summary of serum vitamin B12, MMA and HCY concentrations in 69 samples is shown in Table 2.
Comparison of immunoassay methods and their agreement The regression analysis was performed for each assay separately. Figure 1 illustrates the graphical data of
Table 1 Summary of performance characteristics according to information provided in the vitamin B12 package inserts. Performance characteristics
DxI 800 Unicel
ADVIA Centaur XP
Roche Cobas E601
Architect i2000sr
Principle of the method
Chemiluminescent paramagnetic microparticle immunoassay Traceable to an internal standard manufactured using the purified cyanocobalamin (vitamin B12) < 50 pg/mL 50–1500 pg/mL 4.8–11.4% 126.5–505.1 pg/mL
Chemiluminescent paramagnetic microparticle immunoassay Traceable to an internal standard manufactured using U.S.P. (United States Pharmacopeia) material 45 pg/mL 45–2000 pg/mL 2.4–5% 211–911 pg/mL
Electrochemiluminescent paramagnetic microparticle immunoassay Standardized against the vitamin B12 assay
Chemiluminescent paramagnetic microparticle immunoassay Standardized using cyanocobalamin (USP Reference standard) 83 pg/mL 125–2000 pg/mL 2.7–5.6% 187–883 pg/mL
Traceability
Analytical sensitivity Linearity and precision, % Reference range
30 pg/mL 30–2000 pg/mL 1.7–4% 191–663 pg/mL
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İspir et al.: Comparison of four automated serum vitamin B12 assays 3
Table 2 Summary of serum vitamin B12, MMA and HCY concentrations in 69 samples.
Mean Median SD Concentration range
Vitamin B12, pg/mL DxI 800 Unicel
ADVIA Centaur XP
Roche Cobas E601
Architect i2000sr
347.6 211 409.5 43–2000
522.5 433 447.2 85–2000
472.5 310.4 443.7 78.6–2000
534.8 372 504.8 105–2217
regression analysis of four automated cobalamin assays. Four immunoassay methods were comparable and correlated with each other with slopes ranging from 0.798 to 1.140 and correlation coefficients (r) from 0.898 to 0.987, p < 0.001. However, in the analysis of regression equations, 95% CI for the slopes obtained for Roche Cobas – Architect i2000sr, Architect i2000sr – DxI 800 Unicel and Architect i2000sr – ADVIA Centaur comparisons did not overlap 1.0. Highest correlation was observed between Roche Cobas – Architect i2000sr and lowest correlation was observed between DxI 800 Unicel – ADVIA Centaur. In the absence of a reference method, we used the mean value of results of four automated assays as the best available surrogate for the true value in each sample. Figure 2 illustrates comparison of four methods against the mean value of vitamin B12 results. The mean values of four methods varied from 347.6 to 534.8 pg/mL (Table 2). When compared with the mean of all assays (469.3 pg/mL), the DxI 800 Unicel assay showed the greatest deviation (Figure 2). The mean bias over all samples was highest for the DxI 800 Unicel [–122 pg/mL (–616–125 pg/mL)] and lowest for the Roche Cobas [3 pg/mL (–162–183 pg/mL)]. We also evaluated the agreement between four methods using the CCC (Table 3). Pearson’s coefficients for precision, and the bias correction coefficients were ≥ 0.90 for all immunoassays. However, DxI 800 Unicel shows the lowest Pearson’s coefficients for precision and the bias correction coefficient. We attribute the reason for this discordance in DxI 800 Unicel results to the lowness of reference values used for vitamin B12. Thirty-three results from DxI 800 Unicel, 22 results from Architect i2000sr and ADVIA Centaur analyzers, 27 results from Roche Cobas analyzer were below the currently accepted deficiency level of 200 pg/mL.
Relation of the four methods with functional biomarkers MMA and HCY levels correlated inversely with vitamin B12 results of four analyzers. CLSI document C28A was
MMA, μmol/L
HCY, μmol/L
0.31 0.19 0.34 0.03–1.7
15.13 12.69 8.21 4.95–48.33
used for the determination of MMA reference range and threshold value of 0.3 μmol/L was determined. Comparison of four vitamin B12 methods with MMA is shown on Figure 3. MMA results were correlated with the vitamin B12 results. The correlation coefficients with their 95% CI were –0.337 (–0.531 to –0.109), –0.315 (–0.513 to –0.084), –0.287 (–0.490 to –0.054), –0.275 (–0.480 to –0.041) for ADVIA Centaur, Roche Cobas, Architect i2000sr, DxI 800 Unicel, respectively. We also compared the HCY levels with the vitamin B12 levels. Figure 4 depicts comparison of four vitamin B12 methods with HCY. There was a negative correlation between the vitamin B12 and HCY levels. The correlation coefficients with their 95% CI were –0.448 (–0.619 to –0.237), –0.427 (–0.603 to –0.212), –0.404 (–0.585 to –0.185), –0.341 (–0.535 to –0.113), for ADVIA Centaur, Roche Cobas, Architect i2000sr, DxI 800 Unicel, respectively. These correlation results indicated that there was no statistically significant difference between four methods according to their relationship with MMA and HCY. When the patients with low serum vitamin B12 and high HCY levels are compared to all of the patients with low serum vitamin B12 levels, the ratio was the highest for HCY and ADVIA Centaur comparison, and was the lowest for HCY and DxI 800 Unicel comparison, 16/22 (72.7%) and 17/28 (60.7%), respectively.
Discussion Today, widely used measurement methods quantitate the vitamin B12 levels in serum for the diagnosis of vitamin B12 deficiency. Vitamin B12, the largest of all vitamins, has multiple forms and very low serum levels. It binds to serum proteins tightly [10]. All these properties of vitamin B12, difficulty of producing pure reference materials and the lack of reference method make the standardization of assays more difficult [10]. The standardization of serum folate and vitamin B12 assay methods has not been achieved yet. The World
Brought to you by | University of California - San Francisco Authenticated Download Date | 2/24/15 10:29 AM
4 İspir et al.: Comparison of four automated serum vitamin B12 assays
Slope Intercept r Paired t test
: 1.140 (1.095−1.184) : −3.721 [(−32.369)−(24.927)] : 0.9872 (0.979−0.992) :
