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From Vitamin K Antagonists to Liver International Normalized Ratio: A Historical Journey and Critical Perspective Alexander Gatt, MD1 Dong Chen, MD, PhD2 Rajiv K. Pruthi, MBBS2 Patrick S. Kamath, MD3 Michael D. Leise, MD 3 Aneel A. Ashrani, MD2 William L. Nichols, MD2 Rong He, MD2
University of Malta, Tal-Qroqq, Msida, Malta 2 Special Coagulation Laboratory, Division of Hematopathology, Department of Laboratory Medicine and Pathology, College of Medicine, Mayo Clinic, Rochester, Minnesota 3 Division of Gastroenterology and Hepatology, Department of Internal Medicine, College of Medicine, Mayo Clinic, Rochester, Minnesota
Address for correspondence Rong He, MD, 200 First Street SW, Rochester, MN 55905 (e-mail: [email protected]).
Semin Thromb Hemost 2014;40:845–851.
Abstract Keywords
► ► ► ► ► ► ► ► ►
prothrombin time PT INR ISI liver disease warfarin vitamin K antagonist liver transplant MELD score
Adoption of international normalized ratio (INR) to harmonize prothrombin time has greatly improved the safety and effectiveness of vitamin K antagonists (VKA) oral anticoagulant therapy. INR is also a major laboratory variable in calculating the widely used Model for End-Stage Liver Disease (MELD) score for liver transplant organ prioritization. However, since the conventional INR (INRVKA) is calibrated specifically for VKA patients, its interlaboratory variation has a significant impact on the accuracy of MELD score. Though still requiring further clinical validation in large numbers of waitlisted patients, the alternative liver INR calibrated by using plasma from liver disease patients instead of VKA patients may harmonize the differences and thus more suitable for MELD score calculation. The objective of this article is to review the history of INR, MELD score, and liver INR, and discuss the challenges and solutions of liver INR implementation.
History of the Prothrombin Time Of the many advances in coagulation medicine over the last century, there is no doubt that the development and implementation of oral anticoagulant therapy for the prevention and treatment of thromboembolism constitutes a major landmark. Seminal observations by a veterinarian of sweet clover disease in cows1 led to the discovery of vitamin K antagonists (VKAs), the coumarin class of drugs2. Thereafter VKAs started being used in humans.3 Experience with use of VKAs led to the realization of its narrow therapeutic window and the need for individualized dosing requiring close laboratory monitoring for safe anticoagulation.3,4
published online November 13, 2014
Issue Theme A Short History of Thrombosis and Hemostasis: Part II (40th Year Celebratory Issue); Guest Editor: Emmanuel J. Favaloro, PhD, FFSc (RCPA).
In the 1930s, before the discovery of the VKAs, the prothrombin time (PT) assay had been developed by Quick.5 The utility of the PT assay in monitoring VKAs was soon realized. The principle of the assay was based on tissue thromboplastin (e.g., rabbit brain tissue) mediated initiation of coagulation in recalcified plasma and measuring the clotting time. Subsequent modifications of the thromboplastin improved its performance characteristics.6,7
Birth of International Normalized Ratio In the early 1940s, thromboplastin was prepared in each laboratory from a wide variety of animal tissues (e.g., brain,
Copyright © 2014 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0034-1395160. ISSN 0094-6176.
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1 Department of Pathology, Faculty of Medicine and Surgery,
From Vitamin K Antagonists to Liver INR
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lung, placenta, and heart) and eventually human organs (e.g., cadaveric brain). Differences in preparation methods led to variations in laboratory performance characteristics of the thromboplastin used to measure PT.8 Beginning in the 1950s, given the ease of availability and assumed standardization, commercially manufactured thromboplastins quickly replaced individual laboratory-derived thromboplastins. However, early studies demonstrated apparent geographic variations in VKA dosing and incorrect degrees of anticoagulation. This variation was demonstrated to be due to the differences in activity of the thromboplastins used.9–11 This led to international standardization efforts that demonstrated a more harmonized outcome when the different thromboplastins were compared with a reference thromboplastin reagent and assigned a sensitivity index, the international sensitivity index (ISI). This was used to derive the international normalized ratio (INR) which rapidly gained approval by the international coagulation community.12,13
Math Solves Every Problem, or Does It? A detailed discussion on the assignment/verification of the ISI is beyond the scope of this article. However, some important aspects, as they pertain to the liver ISI, warrant discussion. The INR is the ratio of the patient’s PT and the geometric mean of the PT of normal (minimum 20) individuals (MNPT) raised to the power of the ISI of the thromboplastin reagent being used: INR ¼ (PTPatient/MNPT)ISI The ISI reflects the responsiveness of a thromboplastin relative to the World Health Organization (WHO) standard or international reference preparation (IRP). A thromboplastin with lower ISI (e.g., 1.0) is considered to be a more sensitive (responsive) reagent, and gives longer PT with VKA samples. The ISI value is derived from the WHO calibration procedure by comparing the test thromboplastin against the WHO standard IRP (from the same species origin) with PT results obtained on 20 normal controls and 60 stabilized VKA patients (> 6 weeks on VKA) with an INR range between 1.5 and 4.5. The results are log transformed and plotted on a double-log graph paper with the IRP log-PT on the y-axis and the testing thromboplastin log-PT results on the x-axis. A best straight line is fitted around the points. The ISI of the test reagent is the product of the slope of this orthogonal regression line and the ISI of the IRP. The coefficient of variation (CV) of the slope has to be 3% in order for the ISI value to be acceptable.12,13 The ISI of each reagent is assigned by the manufacturers and may be paired with selected, but not all, instruments. For paired instrument–reagents, local verification of ISI is advisable, whereas for unpaired combinations, assignment of ISI is imperative.14 For either option, the calibration needs to be traceable back to the WHO standard IRP. The classical WHO calibration procedure is a labor-intensive process where PT tests are performed on 20 normal and 60 VKA patients, in parallel with both the IRP (which is in limited supply) and the local thromboplastin using the manual tilt tube method, a skill not widely available. Due to these limitations, this Seminars in Thrombosis & Hemostasis
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procedure is no longer practiced in most of the clinical laboratories.15 The WHO guidelines were revised in 1999.16 Four types of acceptable calibration procedures were described using the IRPs, secondary standards, individual working batches, and local ISI calibration. Although the use of INR has indeed reduced variability in results for VKA patients, variations in the ISI and instrument still result in discrepant results especially with high ISI reagents.17
International Normalized Ratio and Model for End-Stage Liver Disease In an effort to develop tools for assessment of prognosis in patients with end-stage liver disease, Kamath et al developed the Model for End-Stage Liver Disease (MELD) score,18 which is calculated from three laboratory variables. MELD ¼ 3.78 ln (serum bilirubin) þ 11.2 ln (INR) þ 9.57 ln (serum creatinine) þ 6.43 etiology The advantage of the MELD score is that it is comprised only of objective laboratory parameters, namely, the total bilirubin, creatinine, and INR. The MELD score represents an improvement over the Child Turcotte Pugh (CTP) score, which included the subjective variables of hepatic encephalopathy and ascites (CTP variables: total bilirubin, serum albumin, INR or PT increase in seconds as compared with normal donor controls, ascites, and hepatic encephalopathy). The MELD score more accurately ranked patients with cirrhosis at risk for mortality than did the CTP score. Thus, the MELD score supplanted the CTP score as the primary organ allocation criterion for liver transplantation.19,20 After its adoption, clinically relevant variations of MELD scores from the same plasma and serum specimens, resulting in differences of up to 9 MELD points, were noted, largely due to interlaboratory variations in INR.21–23 At present, thromboplastins are routinely calibrated for INR determinations in VKA patients (with a reduction in the activity of vitamin K-dependent factors II, VII, IX, and X, protein C, and protein S, and so on),24 while patients with end-stage liver disease have a different coagulation profile (broad deficiencies of procoagulant, anticoagulant, and fibrinolytic proteins).25–29 In addition, chronic liver disease patients may have a concomitant vitamin K deficiency due to suboptimal dietary intake or cholestasis that may impair fat-soluble vitamin absorption. The latter led to the subsequent exclusion of patients with cholestasis in the development of the liver INR in some studies.30
INRliver and ISIliver Issues discussed earlier in the article has led the investigators to apply the concept of calibration or assignment of ISI (designated ISIliver) of thromboplastins using plasma samples from patients with liver disease rather than VKA patients. In their seminal articles, Tripodi et al and Bellest et al demonstrated the proof of concept that calculating the INR using the ISIliver resulted in an improvement of INR precision in this population of patients.30,31
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The methods used to derive the ISIliver were very similar to the WHO calibration procedure described earlier for VKA patients, with the exception that the VKA plasma samples were substituted by the plasma samples from patients with liver disease. Around 60 liver disease patients were selected in both studies for the alternative calibration. The same patient cohort and a separate cohort of 34 liver disease patients were evaluated for liver INR and MELD scores in the Tripodi and Bellest studies, respectively. Tripodi et al chose their cohort depending on the CTP score while the French group used a different system based on the PT ratio as well as factor V levels. The etiology of liver cirrhosis/failure varied and included alcoholic liver disease, viral hepatitis, and cryptogenic cirrhosis. Patients with cholestatic jaundice were excluded in the Tripodi et al study. Samples from patients were stored at 70°C and not tested fresh in both studies. Commercial tissue derived and recombinant thromboplastin reagents from different manufacturers were analyzed on an Electra MLA1600 coagulometer and ACL TOP coagulometer (Instrumentation Laboratory, Lexington, MA) in the Tripodi et al and Bellest et al studies, respectively. In the first study, the tested thromboplastins were calibrated against a secondary standard run on the same Electra MLA1600 coagulometer, while in the Bellest et al study the choice of thromboplastins was run on an ACL TOP and calibrated against the WHO IRP (rTF/95) using the manual tilt-tube technique. The data demonstrated significant variation of INR and MELD scores among the testing thromboplastins when the conventional ISI, that is, ISIVKA was used in the calculation. This variability was significantly reduced upon replacement of the ISIVKA with the ISIliver. In the Tripodi et al study, the mean between-thromboplastin CV of INRVKA of 8.5% was reduced to 1% with the INRliver; the between-thromboplastin CV of MELDVKA scores dropped from 6.7 to 0.8% with the MELDliver score. Bellest et al observed that in the 34 liver disease patients studied, only INRliver eliminated variability in the INR results between different thromboplastins, whereas PT ratio and INRVKA did not (3, 9, and 16%, between-thromboplastin CV of INRliver, PT ratio, and INRVKA, respectively). The ISIs of the various thromboplastins were brought closer together by the ISIliver calibration (►Table 1) and most were close to 1.0 (ISIliver vs. ISIVKA, 0.84–1.14 vs. 0.87– 1.77, Tripodi et al 0.70–0.98 vs. 0.80–1.72 Bellest et al). The six rabbit brain thromboplastins demonstrated the greatest difference between the ISIliver and ISIVKA (42.2–70.4%) and the human recombinant thromboplastin RecombiPlasTin (Instrumentation Laboratory Lexington, MA) showed the smallest difference (8.6% in the Tripodi et al study and 2.4% in the Bellest et al study).30,31 Little difference (11.8%) between the two ISIs was seen in another human recombinant thromboplastin, Innovin (Dade Behring, Marburg, Germany). Thrombotest (Axis Shield, Oslo, Norway), an ox brain extract supplemented with optimal amount of fibrinogen and factor V also showed a minimal difference of 5.7%. Using three commercial thromboplastins, Innovin, a recombinant human thromboplastin, Thromborel S (Dade Behring), a human placenta thromboplastin, and
Gatt et al.
Thromboplastin C (Dade Behring), a rabbit brain thromboplastin, Sermon et al demonstrated similar discrepancies between the INRVKA and INRliver32 again confirming the impact of ISIVKA and ISIliver on the MELD score. Among the three thromboplastins tested, Innovin demonstrated a 7.3% change of the ISI from 1.03 (ISIVKA) to 0.96 (ISIliver) with 60, 38, and 2% of liver disease patient samples showing 0, 1, and 2 points change in MELD scores, respectively. MELD score changes from 4 to 5 points were only seen with the other two thromboplastins tested. In 2009 Burroughs et al also reported similar INRs with the ISIliver and ISIVKA seen with RecombiPlasTin.33 Tripodi et al also took this ISIliver concept a step further by studying whether it can actually be incorporated into pointof-care (POC) coagulation testing devices.34 They used two POC INR analyzers, Coaguchek S and the Coaguchek XS (Roche Diagnostics, Fishers, IN), which use a rabbit and a human recombinant thromboplastin, respectively. A nonanticoagulated venous blood sample was used to measure the INR on both machines, whereas a citrated plasma sample was used to measure the PT using WHO IRP rTF/95 by manual tilt-tube method. Again, they derived the ISIliver for both devices and showed that using the ISIliver, the INR results moved closer to their respective line of identity when compared with the usual ISIVKA in cirrhotic patients. From the above studies, it is clear that in most instances, the alternative calibration of ISIliver tightens and standardizes INRs among different thromboplastins in patients with liver disease. In addition, the differences between INRVKA and INRliver are small when human thromboplastins are used in comparison to the rabbit brain thromboplastins (►Table 1). This is especially true for the two human recombinant thromboplastins, RecombiPlasTin and Innovin, with ISIVKA ranging from 0.78 to 1.03; ISI differences fell between 2.4 and 11.8%. As expected, the rabbit brain thromboplastins came with higher ISIVKA ranging 1.05 to 1.91, and 67% of them showed a marked ISI difference of at least 40%. This means that they would result in greater difference between MELDVKA and MELDliver scores, whereas the MELD score difference can be negligible with human recombinant thromboplastins, RecombiPlasTin and Innovin. Of interest, in the original Mayo Clinic MELD score development study published in 2001, the PT assays were performed using Innovin. This human recombinant thromboplastin had an ISI close to 1.0 and demonstrated minimal difference between the ISIliver and ISIVKA as well as the following INRs and MELD scores,30,32 and therefore the classical INRVKA of Innovin should be interchangeable with its INRliver.
Challenges and Solutions of INRliver Implementation The studies reviewed above demonstrated that the INRliver is more suitable than the conventional INRVKA for MELD score calculation and, therefore, liver transplant prioritization, though this requires further validation in large numbers of waitlisted patients. It has already been 7 years since the publication of the two seminal papers on liver INR, but we are still far from wide adoption of this system within our Seminars in Thrombosis & Hemostasis
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From Vitamin K Antagonists to Liver INR
From Vitamin K Antagonists to Liver INR
Gatt et al.
Table 1 Summary of ISIVKA and ISIliver comparison of various thromboplastins reported in literature Thromboplastin
Manufacturer
Coagulometer
Origin
ISIVKA
ISIliver
Difference (%)
References
RecombiPlasTin
Instrumentation Laboratory
Electra MLA 1600
Human recombinant
0.87
0.952
8.6
Tripodi et al30
RecombiPlasTin
Instrumentation Laboratory
ACL TOP
Human recombinant
0.83
0.85
2.4
Bellest et al31
RecombiPlasTin
Instrumentation Laboratory
ACL TOP
Human recombinant
0.78
0.84
7.1
Riddell et al33
Innovin
Dade Behring
Electra MLA 1600
Human recombinant
0.945
0.845
11.8
Tripodi et al30
Innovin
Dade Behring
Sysmex CA-1500
Human recombinant
1.03
0.96
7.3
Sermon et al32
Innovin
Dade Behring
ACL TOP
Human recombinant
0.93
1.03
9.7
Riddell et al33
Thromborel S
Dade Behring
Electra MLA 1600
Human placenta
1.111
0.886
25.4
Tripodi et al30
Thromborel S
Dade Behring
Sysmex CA-1500
Human Placenta
0.98
0.82
12.2
Sermon et al32
Thromborel S
Dade Behring
ACL TOP
Human placenta
1.03
0.84
22.6
Bellest et al31
Neoplastin Plus
Diagnostica Stago
Electra MLA 1600
Rabbit brain
1.297
0.844
53.7
Tripodi et al30
Neoplastin CL
Diagnostica Stago
ACL TOP
Rabbit brain
1.67
0.98
70.4
Bellest et al31
Neoplastin CL Plus
Diagnostica Stago
ACL TOP
Rabbit brain
1.25
1.17
6.8
Riddell et al33
PT HS
Instrumentation Laboratory
Electra MLA 1600
Rabbit brain
1.463
1.029
42.2
Tripodi et al30
PT Fib HS Plus
Instrumentation Laboratory
ACL TOP
Rabbit brain
1.13
0.95
18.9
Riddell et al33
Simplastin Excel
Biomerieux
Electra MLA 1600
Rabbit brain
1.768
1.141
55
Tripodi et al30
Simplastin Excel
Biomerieux
ACL TOP
Rabbit brain
1.54
0.94
63.8
Bellest et al31
Simplastin Excel S
Biomerieux
ACL TOP
Rabbit brain
1.05
0.7
50
Bellest et al31
Thromboplastin C
Dade Behring
Sysmex CA-1500
Rabbit brain
1.91
1.64
16.5
Sermon et al32
Thrombotest
Axis Shield
Electra MLA 1600
Ox brain combined
0.881
0.934
5.7
Tripodi et al30
routine coagulation laboratories. Multiple modes of liver INR implementation have been proposed, however, they are also fraught with logistic challenges.22,35,36 Since the establishment of ISIliver requires calibration against the WHO IRP using manual tilt-tube methodology as well as 60 patient plasma samples spanning the spectrum of liver disease severity, it is impractical for most local laboratories to carry out this demanding task. Therefore, it was proposed that the calibration could be performed by the thromboplastin manufacturers. In this case, the packet inserts should include two ISIs, the ISIVKA and the ISIliver, and the coagulometer instrument(s) for which they were validated. Although possible, this proposal adds expenses and effort for the manufacturers and will take time before it can be fully endorsed and implemented. Furthermore, the concurrent existence of two similar INR systems will undoubtedly create confusion in the coagulation diagnostic testing laboratories and for the ordering physicians. Bellest et al advocated the use of standard pools of plasma derived from patients with liver disease that individual Seminars in Thrombosis & Hemostasis
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laboratories could use to derive the ISIliver, similar to the certified plasmas in the local verification/calibration of ISIVKA.31 However, large amounts of plasma would have to be collected, processed, and coordinated for distribution to individual coagulation laboratories. Frozen plasma is not ideal as a long-term reference material due to stability issues and lyophilized plasma although stable may alter the functionality of some coagulation factors. Another option is to have a centralized approach with only a few reference laboratories performing the liver INR testing for patients on the liver transplant waiting list, optimally using the same thromboplastin/instrument combination and establishing the ISIliver following the calibration procedure described by the two seminal papers described earlier. Ideally, serum testing of bilirubin and creatinine should also be performed in the same reference laboratory, since these two assays are also not devoid of laboratory variations. However, such centralization would be difficult to implement in many areas and countries and may be impractical for very sick patients whose MELD score needs to be recertified on a daily basis.
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Another way, as suggested by the Scientific and Standardization Committee (SSC) on anticoagulation control of the International Society of Thrombosis and Hemostasis (ISTH),37 is to choose reagents that give similar ISI values (CV < 10%) irrespective of whether such calibration is derived from the plasma samples of patients on VKA therapy or those with liver disease. From the Tripodi et al and Bellest et al studies as well as Burroughs et al’s research, it is apparent that certain reagents are equally sensitive to both VKA therapy and liver disease plasmas.30,31,33 Two promising candidates are the human recombinant thromboplastins, Innovin and RecombiPlasTin. Innovin is the thromboplastin used in the original Mayo Clinic derivation and validation of MELD score in endstage liver disease patients, and the most studied. RecombiPlasTin was tested side by side with Innovin in the Tripodi et al study30 and showed a negligible difference from Innovin in terms of INRVKA and MELDVKA score, as well as the standardized INRliver and MELDliver score. In such scenarios, it seems that one reagent with one assigned ISI can be utilized for testing plasmas from both types of patients. This might be an ideal solution since the same ISI would then be used for both indications, and obviates the need to set up a new “INR.” A logistic concern for this option is that one cannot compel laboratories to purchase a particular reagent. In addition, further validation is also required. Removing INR from the MELD score calculation has also been suggested, and preliminary results support the validity of this approach.38 However, the study population was limited to hepatitis C and/or alcoholic cirrhosis patients, and the impact of omitting the INR in the MELD score calculation in liver disease patients with other etiologies such as biliary cirrhosis, autoimmune hepatitis or other metabolic disorders was not assessed. There are also proposals for replacing INR with other parameters of hepatic synthetic reserve, such as measurements of plasma coagulation factors V or VII.39 However, such factor assays are relatively expensive, not routinely tested in patients with liver disease, and generally lack standardization. Recently, a revised MELD score found that capping the INR value at 3 led to the improved waitlist mortality prediction, and theoretically this INR cap could reduce some of the impact of the variability in INRvka on the calculated MELD score.40 Finally, the Quick PT is not the only kind of PT test available. Although less commonly used, the Owren PT system offers an alternative. The Owren PT reagents are supplemented with bovine factor V and fibrinogen, two clotting factors that are not affected by VKAs but which are variably affected in liver disease patients.41 Tripodi et al had included one Owren reagent, Thrombotest, in their study and observed minimal difference between the ISIliver and ISIVKA from this thromboplastin. The two ISI values were also very close to that of the RecombiPlasTin.30 Magnusson et al went a step further and studied the Owren method in a multilaboratory nationwide study.42 They showed that using three Owren reagents, namely, SPA þ, Owren PT or the Nycotest PT, there was no difference in the ISI whether it was derived from patients on VKA therapy or from patients with liver disease. However, its appropriateness to be used in
Gatt et al.
the MELD score calculation and subsequent liver transplant prioritization remains controversial, as the deficiency of factor V and fibrinogen associated with hepatic synthetic dysfunction would be masked by the system.43 Factor V is believed to be a reliable marker of severity of liver disease and plasma levels tend to decrease with worsening liver failure. Fibrinogen plasma levels also decrease, especially with very severe liver disease, albeit to a lesser extent than factor V. Although in the Tripodi et al study the Thrombotest was as sensitive to liver disease-induced coagulopathy as was RecombiPlasTin, this study only included 57 liver cirrhosis patients with 18 of them classified as CTP class C, the class with the most severe liver disease in the three-tier grading system. The validity of the above findings and the use of Owren reagent in the MELD score calculation would require further evaluation in larger cohorts of chronic liver disease patients, especially large population of patients with more severe liver disease. Future efforts for optimizing the INR in patients with liver disease would require testing in large cohorts of waitlisted patients to demonstrate validated model calibration and discrimination using INRliver as seen in newer iterations of the MELD model.40,44 It is not practical to expect local coagulation laboratories to perform such clinical validation. Such validation studies could be done by selecting laboratories and medical centers with extensive expertise in coagulation laboratory testing and liver transplantation. Information from these studies should be documented and shared publicly.
Conclusions In 2007 an alternative INR system was described for patients with liver disease to standardize the reporting of INR in this patient population. This system minimizes the interlaboratory variations in the MELD score and ensures organ allocations for liver transplantation to the sickest patients. However, the introduction of the INRliver has been hampered with multiple practical and logistical issues and as yet is not being widely adopted. As Marlar predicted soon after its description, both laboratorians and clinicians would have to work together if there was going to be any change toward liver INR.36 The SSC on anticoagulation control of the ISTH has made recommendations for potential use of INRliver testing and reporting.37 It was recommended that the INRVKA should not be used interchangeably for patients with coagulopathy of liver disease. For laboratories using a particular thromboplastin reagent that has been proven to yield similar ISIs whether for patients on VKA therapy or those with liver disease one thromboplastin and ISI can be used for both indications. In general, thromboplastins that have consistently demonstrated close similarity between the ISIliver and ISIVKA should be the preferred thromboplastins used for measurement of INR for calculation of MELD scores. Importantly, although the INRliver has been demonstrated to result in reduced variability of MELD scores, compared with the INRVKA, its effect on mortality prediction in patients with liver disease remains to be demonstrated. Seminars in Thrombosis & Hemostasis
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From Vitamin K Antagonists to Liver INR
From Vitamin K Antagonists to Liver INR
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(MELD) scores for patients listed for liver transplantation. Liver Transpl 2004;10(8):995–1000 Porte RJ, Lisman T, Tripodi A, Caldwell SH, Trotter JF; Coagulation in Liver Disease Study Group. The International Normalized Ratio (INR) in the MELD score: problems and solutions. Am J Transplant 2010;10(6):1349–1353 Trotter JF, Olson J, Lefkowitz J, Smith AD, Arjal R, Kenison J. Changes in international normalized ratio (INR) and model for endstage liver disease (MELD) based on selection of clinical laboratory. Am J Transplant 2007;7(6):1624–1628 Gatt A. Global Assays of Coagulation in Anticoagulation, Thrombotic Disorders and Liver Disease. Sheffield, UK: University of Sheffield; 2012 Gatt A, Riddell A, Calvaruso V, Tuddenham EG, Makris M, Burroughs AK. Enhanced thrombin generation in patients with cirrhosisinduced coagulopathy. J Thromb Haemost 2010;8(9):1994–2000 Tripodi A, Salerno F, Chantarangkul V, et al. Evidence of normal thrombin generation in cirrhosis despite abnormal conventional coagulation tests. Hepatology 2005;41(3):553–558 Tripodi A, Mannucci PM. The coagulopathy of chronic liver disease. N Engl J Med 2011;365(2):147–156 Wada H, Usui M, Sakuragawa N. Hemostatic abnormalities and liver diseases. Semin Thromb Hemost 2008;34(8):772–778 Francis JL, Armstrong DJ. Acquired dysfibrinogenaemia in liver disease. J Clin Pathol 1982;35(6):667–672 Tripodi A, Chantarangkul V, Primignani M, et al. The international normalized ratio calibrated for cirrhosis (INR(liver)) normalizes prothrombin time results for model for end-stage liver disease calculation. Hepatology 2007;46(2):520–527 Bellest L, Eschwège V, Poupon R, Chazouillères O, Robert A. A modified international normalized ratio as an effective way of prothrombin time standardization in hepatology. Hepatology 2007;46(2):528–534 Sermon AM, Smith JM, Maclean R, Kitchen S. An International Sensitivity Index (ISI) derived from patients with abnormal liver function improves agreement between INRs determined with different reagents. Thromb Haemost 2010;103(4):757–765 Riddell AF, Calvaruso V, Tuddenham EG, Burroughs A, Gatt A. A pragmatic approach for the measurement of the INR in liver patients. J Thromb Haemost 2009;7:abstract 10–6 Tripodi A, Chantarangkul V, Primignani M, et al. Point-of-care coagulation monitors calibrated for the international normalized ratio for cirrhosis (INRliver) can help to implement the INRliver for the calculation of the MELD score. J Hepatol 2009;51(2):288–295 Tripodi A. How to implement the modified international normalized ratio for cirrhosis (INR(liver)) for model for end-stage liver disease calculation. Hepatology 2008;47(4):1423–1424, author reply 1424 Marlar RA. Determining the model for end-stage liver disease with better accuracy: neutralizing the international normalized ratio pitfalls. Hepatology 2007;46(2):295–296 Tripodi A, Baglin T, Robert A, Kitchen S, Lisman T, Trotter JF; Subcommittee on Control of Anticoagulation of the Scientific and Standardisation Committee of the International Society on Thrombosis and Haemostasis. Reporting prothrombin time results as international normalized ratios for patients with chronic liver disease. J Thromb Haemost 2010;8(6):1410–1412 Heuman DM, Mihas AA, Habib A, et al. MELD-XI: a rational approach to “sickest first” liver transplantation in cirrhotic patients requiring anticoagulant therapy. Liver Transpl 2007;13(1):30–37 Izumi S, Langley PG, Wendon J, et al. Coagulation factor V levels as a prognostic indicator in fulminant hepatic failure. Hepatology 1996;23(6):1507–1511 Leise MD, Kim WR, Kremers WK, Larson JJ, Benson JT, Therneau TM. A revised model for end-stage liver disease optimizes prediction of mortality among patients awaiting liver transplantation. Gastroenterology 2011;140(7):1952–1960
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43 Gatt A, Burroughs AK. The Owren INR in liver disease: a
lant therapy. Lancet 1959;2(7106):754–758 42 Magnusson M, Sten-Linder M, Bergquist A, et al. The international normalized ratio according to Owren in liver disease: interlaboratory assessment and determination of international sensitivity index. Thromb Res 2013;132(3):346–351
way towards standardisation? Thromb Res 2013;132(3): 327–328 44 Kim WR, Biggins SW, Kremers WK, et al. Hyponatremia and mortality among patients on the liver-transplant waiting list. N Engl J Med 2008;359(10):1018–1026
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41 Owren PA. Thrombotest. A new method for controlling anticoagu-
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From Vitamin K Antagonists to Liver International Normalized Ratio: A Historical Journey and Critical Perspective Alexander Gatt, MD1 Dong Chen, MD, PhD2 Rajiv K. Pruthi, MBBS2 Patrick S. Kamath, MD3 Michael D. Leise, MD 3 Aneel A. Ashrani, MD2 William L. Nichols, MD2 Rong He, MD2
University of Malta, Tal-Qroqq, Msida, Malta 2 Special Coagulation Laboratory, Division of Hematopathology, Department of Laboratory Medicine and Pathology, College of Medicine, Mayo Clinic, Rochester, Minnesota 3 Division of Gastroenterology and Hepatology, Department of Internal Medicine, College of Medicine, Mayo Clinic, Rochester, Minnesota
Address for correspondence Rong He, MD, 200 First Street SW, Rochester, MN 55905 (e-mail: [email protected]).
Semin Thromb Hemost 2014;40:845–851.
Abstract Keywords
► ► ► ► ► ► ► ► ►
prothrombin time PT INR ISI liver disease warfarin vitamin K antagonist liver transplant MELD score
Adoption of international normalized ratio (INR) to harmonize prothrombin time has greatly improved the safety and effectiveness of vitamin K antagonists (VKA) oral anticoagulant therapy. INR is also a major laboratory variable in calculating the widely used Model for End-Stage Liver Disease (MELD) score for liver transplant organ prioritization. However, since the conventional INR (INRVKA) is calibrated specifically for VKA patients, its interlaboratory variation has a significant impact on the accuracy of MELD score. Though still requiring further clinical validation in large numbers of waitlisted patients, the alternative liver INR calibrated by using plasma from liver disease patients instead of VKA patients may harmonize the differences and thus more suitable for MELD score calculation. The objective of this article is to review the history of INR, MELD score, and liver INR, and discuss the challenges and solutions of liver INR implementation.
History of the Prothrombin Time Of the many advances in coagulation medicine over the last century, there is no doubt that the development and implementation of oral anticoagulant therapy for the prevention and treatment of thromboembolism constitutes a major landmark. Seminal observations by a veterinarian of sweet clover disease in cows1 led to the discovery of vitamin K antagonists (VKAs), the coumarin class of drugs2. Thereafter VKAs started being used in humans.3 Experience with use of VKAs led to the realization of its narrow therapeutic window and the need for individualized dosing requiring close laboratory monitoring for safe anticoagulation.3,4
published online November 13, 2014
Issue Theme A Short History of Thrombosis and Hemostasis: Part II (40th Year Celebratory Issue); Guest Editor: Emmanuel J. Favaloro, PhD, FFSc (RCPA).
In the 1930s, before the discovery of the VKAs, the prothrombin time (PT) assay had been developed by Quick.5 The utility of the PT assay in monitoring VKAs was soon realized. The principle of the assay was based on tissue thromboplastin (e.g., rabbit brain tissue) mediated initiation of coagulation in recalcified plasma and measuring the clotting time. Subsequent modifications of the thromboplastin improved its performance characteristics.6,7
Birth of International Normalized Ratio In the early 1940s, thromboplastin was prepared in each laboratory from a wide variety of animal tissues (e.g., brain,
Copyright © 2014 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0034-1395160. ISSN 0094-6176.
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1 Department of Pathology, Faculty of Medicine and Surgery,
From Vitamin K Antagonists to Liver INR
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lung, placenta, and heart) and eventually human organs (e.g., cadaveric brain). Differences in preparation methods led to variations in laboratory performance characteristics of the thromboplastin used to measure PT.8 Beginning in the 1950s, given the ease of availability and assumed standardization, commercially manufactured thromboplastins quickly replaced individual laboratory-derived thromboplastins. However, early studies demonstrated apparent geographic variations in VKA dosing and incorrect degrees of anticoagulation. This variation was demonstrated to be due to the differences in activity of the thromboplastins used.9–11 This led to international standardization efforts that demonstrated a more harmonized outcome when the different thromboplastins were compared with a reference thromboplastin reagent and assigned a sensitivity index, the international sensitivity index (ISI). This was used to derive the international normalized ratio (INR) which rapidly gained approval by the international coagulation community.12,13
Math Solves Every Problem, or Does It? A detailed discussion on the assignment/verification of the ISI is beyond the scope of this article. However, some important aspects, as they pertain to the liver ISI, warrant discussion. The INR is the ratio of the patient’s PT and the geometric mean of the PT of normal (minimum 20) individuals (MNPT) raised to the power of the ISI of the thromboplastin reagent being used: INR ¼ (PTPatient/MNPT)ISI The ISI reflects the responsiveness of a thromboplastin relative to the World Health Organization (WHO) standard or international reference preparation (IRP). A thromboplastin with lower ISI (e.g., 1.0) is considered to be a more sensitive (responsive) reagent, and gives longer PT with VKA samples. The ISI value is derived from the WHO calibration procedure by comparing the test thromboplastin against the WHO standard IRP (from the same species origin) with PT results obtained on 20 normal controls and 60 stabilized VKA patients (> 6 weeks on VKA) with an INR range between 1.5 and 4.5. The results are log transformed and plotted on a double-log graph paper with the IRP log-PT on the y-axis and the testing thromboplastin log-PT results on the x-axis. A best straight line is fitted around the points. The ISI of the test reagent is the product of the slope of this orthogonal regression line and the ISI of the IRP. The coefficient of variation (CV) of the slope has to be 3% in order for the ISI value to be acceptable.12,13 The ISI of each reagent is assigned by the manufacturers and may be paired with selected, but not all, instruments. For paired instrument–reagents, local verification of ISI is advisable, whereas for unpaired combinations, assignment of ISI is imperative.14 For either option, the calibration needs to be traceable back to the WHO standard IRP. The classical WHO calibration procedure is a labor-intensive process where PT tests are performed on 20 normal and 60 VKA patients, in parallel with both the IRP (which is in limited supply) and the local thromboplastin using the manual tilt tube method, a skill not widely available. Due to these limitations, this Seminars in Thrombosis & Hemostasis
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procedure is no longer practiced in most of the clinical laboratories.15 The WHO guidelines were revised in 1999.16 Four types of acceptable calibration procedures were described using the IRPs, secondary standards, individual working batches, and local ISI calibration. Although the use of INR has indeed reduced variability in results for VKA patients, variations in the ISI and instrument still result in discrepant results especially with high ISI reagents.17
International Normalized Ratio and Model for End-Stage Liver Disease In an effort to develop tools for assessment of prognosis in patients with end-stage liver disease, Kamath et al developed the Model for End-Stage Liver Disease (MELD) score,18 which is calculated from three laboratory variables. MELD ¼ 3.78 ln (serum bilirubin) þ 11.2 ln (INR) þ 9.57 ln (serum creatinine) þ 6.43 etiology The advantage of the MELD score is that it is comprised only of objective laboratory parameters, namely, the total bilirubin, creatinine, and INR. The MELD score represents an improvement over the Child Turcotte Pugh (CTP) score, which included the subjective variables of hepatic encephalopathy and ascites (CTP variables: total bilirubin, serum albumin, INR or PT increase in seconds as compared with normal donor controls, ascites, and hepatic encephalopathy). The MELD score more accurately ranked patients with cirrhosis at risk for mortality than did the CTP score. Thus, the MELD score supplanted the CTP score as the primary organ allocation criterion for liver transplantation.19,20 After its adoption, clinically relevant variations of MELD scores from the same plasma and serum specimens, resulting in differences of up to 9 MELD points, were noted, largely due to interlaboratory variations in INR.21–23 At present, thromboplastins are routinely calibrated for INR determinations in VKA patients (with a reduction in the activity of vitamin K-dependent factors II, VII, IX, and X, protein C, and protein S, and so on),24 while patients with end-stage liver disease have a different coagulation profile (broad deficiencies of procoagulant, anticoagulant, and fibrinolytic proteins).25–29 In addition, chronic liver disease patients may have a concomitant vitamin K deficiency due to suboptimal dietary intake or cholestasis that may impair fat-soluble vitamin absorption. The latter led to the subsequent exclusion of patients with cholestasis in the development of the liver INR in some studies.30
INRliver and ISIliver Issues discussed earlier in the article has led the investigators to apply the concept of calibration or assignment of ISI (designated ISIliver) of thromboplastins using plasma samples from patients with liver disease rather than VKA patients. In their seminal articles, Tripodi et al and Bellest et al demonstrated the proof of concept that calculating the INR using the ISIliver resulted in an improvement of INR precision in this population of patients.30,31
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The methods used to derive the ISIliver were very similar to the WHO calibration procedure described earlier for VKA patients, with the exception that the VKA plasma samples were substituted by the plasma samples from patients with liver disease. Around 60 liver disease patients were selected in both studies for the alternative calibration. The same patient cohort and a separate cohort of 34 liver disease patients were evaluated for liver INR and MELD scores in the Tripodi and Bellest studies, respectively. Tripodi et al chose their cohort depending on the CTP score while the French group used a different system based on the PT ratio as well as factor V levels. The etiology of liver cirrhosis/failure varied and included alcoholic liver disease, viral hepatitis, and cryptogenic cirrhosis. Patients with cholestatic jaundice were excluded in the Tripodi et al study. Samples from patients were stored at 70°C and not tested fresh in both studies. Commercial tissue derived and recombinant thromboplastin reagents from different manufacturers were analyzed on an Electra MLA1600 coagulometer and ACL TOP coagulometer (Instrumentation Laboratory, Lexington, MA) in the Tripodi et al and Bellest et al studies, respectively. In the first study, the tested thromboplastins were calibrated against a secondary standard run on the same Electra MLA1600 coagulometer, while in the Bellest et al study the choice of thromboplastins was run on an ACL TOP and calibrated against the WHO IRP (rTF/95) using the manual tilt-tube technique. The data demonstrated significant variation of INR and MELD scores among the testing thromboplastins when the conventional ISI, that is, ISIVKA was used in the calculation. This variability was significantly reduced upon replacement of the ISIVKA with the ISIliver. In the Tripodi et al study, the mean between-thromboplastin CV of INRVKA of 8.5% was reduced to 1% with the INRliver; the between-thromboplastin CV of MELDVKA scores dropped from 6.7 to 0.8% with the MELDliver score. Bellest et al observed that in the 34 liver disease patients studied, only INRliver eliminated variability in the INR results between different thromboplastins, whereas PT ratio and INRVKA did not (3, 9, and 16%, between-thromboplastin CV of INRliver, PT ratio, and INRVKA, respectively). The ISIs of the various thromboplastins were brought closer together by the ISIliver calibration (►Table 1) and most were close to 1.0 (ISIliver vs. ISIVKA, 0.84–1.14 vs. 0.87– 1.77, Tripodi et al 0.70–0.98 vs. 0.80–1.72 Bellest et al). The six rabbit brain thromboplastins demonstrated the greatest difference between the ISIliver and ISIVKA (42.2–70.4%) and the human recombinant thromboplastin RecombiPlasTin (Instrumentation Laboratory Lexington, MA) showed the smallest difference (8.6% in the Tripodi et al study and 2.4% in the Bellest et al study).30,31 Little difference (11.8%) between the two ISIs was seen in another human recombinant thromboplastin, Innovin (Dade Behring, Marburg, Germany). Thrombotest (Axis Shield, Oslo, Norway), an ox brain extract supplemented with optimal amount of fibrinogen and factor V also showed a minimal difference of 5.7%. Using three commercial thromboplastins, Innovin, a recombinant human thromboplastin, Thromborel S (Dade Behring), a human placenta thromboplastin, and
Gatt et al.
Thromboplastin C (Dade Behring), a rabbit brain thromboplastin, Sermon et al demonstrated similar discrepancies between the INRVKA and INRliver32 again confirming the impact of ISIVKA and ISIliver on the MELD score. Among the three thromboplastins tested, Innovin demonstrated a 7.3% change of the ISI from 1.03 (ISIVKA) to 0.96 (ISIliver) with 60, 38, and 2% of liver disease patient samples showing 0, 1, and 2 points change in MELD scores, respectively. MELD score changes from 4 to 5 points were only seen with the other two thromboplastins tested. In 2009 Burroughs et al also reported similar INRs with the ISIliver and ISIVKA seen with RecombiPlasTin.33 Tripodi et al also took this ISIliver concept a step further by studying whether it can actually be incorporated into pointof-care (POC) coagulation testing devices.34 They used two POC INR analyzers, Coaguchek S and the Coaguchek XS (Roche Diagnostics, Fishers, IN), which use a rabbit and a human recombinant thromboplastin, respectively. A nonanticoagulated venous blood sample was used to measure the INR on both machines, whereas a citrated plasma sample was used to measure the PT using WHO IRP rTF/95 by manual tilt-tube method. Again, they derived the ISIliver for both devices and showed that using the ISIliver, the INR results moved closer to their respective line of identity when compared with the usual ISIVKA in cirrhotic patients. From the above studies, it is clear that in most instances, the alternative calibration of ISIliver tightens and standardizes INRs among different thromboplastins in patients with liver disease. In addition, the differences between INRVKA and INRliver are small when human thromboplastins are used in comparison to the rabbit brain thromboplastins (►Table 1). This is especially true for the two human recombinant thromboplastins, RecombiPlasTin and Innovin, with ISIVKA ranging from 0.78 to 1.03; ISI differences fell between 2.4 and 11.8%. As expected, the rabbit brain thromboplastins came with higher ISIVKA ranging 1.05 to 1.91, and 67% of them showed a marked ISI difference of at least 40%. This means that they would result in greater difference between MELDVKA and MELDliver scores, whereas the MELD score difference can be negligible with human recombinant thromboplastins, RecombiPlasTin and Innovin. Of interest, in the original Mayo Clinic MELD score development study published in 2001, the PT assays were performed using Innovin. This human recombinant thromboplastin had an ISI close to 1.0 and demonstrated minimal difference between the ISIliver and ISIVKA as well as the following INRs and MELD scores,30,32 and therefore the classical INRVKA of Innovin should be interchangeable with its INRliver.
Challenges and Solutions of INRliver Implementation The studies reviewed above demonstrated that the INRliver is more suitable than the conventional INRVKA for MELD score calculation and, therefore, liver transplant prioritization, though this requires further validation in large numbers of waitlisted patients. It has already been 7 years since the publication of the two seminal papers on liver INR, but we are still far from wide adoption of this system within our Seminars in Thrombosis & Hemostasis
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From Vitamin K Antagonists to Liver INR
From Vitamin K Antagonists to Liver INR
Gatt et al.
Table 1 Summary of ISIVKA and ISIliver comparison of various thromboplastins reported in literature Thromboplastin
Manufacturer
Coagulometer
Origin
ISIVKA
ISIliver
Difference (%)
References
RecombiPlasTin
Instrumentation Laboratory
Electra MLA 1600
Human recombinant
0.87
0.952
8.6
Tripodi et al30
RecombiPlasTin
Instrumentation Laboratory
ACL TOP
Human recombinant
0.83
0.85
2.4
Bellest et al31
RecombiPlasTin
Instrumentation Laboratory
ACL TOP
Human recombinant
0.78
0.84
7.1
Riddell et al33
Innovin
Dade Behring
Electra MLA 1600
Human recombinant
0.945
0.845
11.8
Tripodi et al30
Innovin
Dade Behring
Sysmex CA-1500
Human recombinant
1.03
0.96
7.3
Sermon et al32
Innovin
Dade Behring
ACL TOP
Human recombinant
0.93
1.03
9.7
Riddell et al33
Thromborel S
Dade Behring
Electra MLA 1600
Human placenta
1.111
0.886
25.4
Tripodi et al30
Thromborel S
Dade Behring
Sysmex CA-1500
Human Placenta
0.98
0.82
12.2
Sermon et al32
Thromborel S
Dade Behring
ACL TOP
Human placenta
1.03
0.84
22.6
Bellest et al31
Neoplastin Plus
Diagnostica Stago
Electra MLA 1600
Rabbit brain
1.297
0.844
53.7
Tripodi et al30
Neoplastin CL
Diagnostica Stago
ACL TOP
Rabbit brain
1.67
0.98
70.4
Bellest et al31
Neoplastin CL Plus
Diagnostica Stago
ACL TOP
Rabbit brain
1.25
1.17
6.8
Riddell et al33
PT HS
Instrumentation Laboratory
Electra MLA 1600
Rabbit brain
1.463
1.029
42.2
Tripodi et al30
PT Fib HS Plus
Instrumentation Laboratory
ACL TOP
Rabbit brain
1.13
0.95
18.9
Riddell et al33
Simplastin Excel
Biomerieux
Electra MLA 1600
Rabbit brain
1.768
1.141
55
Tripodi et al30
Simplastin Excel
Biomerieux
ACL TOP
Rabbit brain
1.54
0.94
63.8
Bellest et al31
Simplastin Excel S
Biomerieux
ACL TOP
Rabbit brain
1.05
0.7
50
Bellest et al31
Thromboplastin C
Dade Behring
Sysmex CA-1500
Rabbit brain
1.91
1.64
16.5
Sermon et al32
Thrombotest
Axis Shield
Electra MLA 1600
Ox brain combined
0.881
0.934
5.7
Tripodi et al30
routine coagulation laboratories. Multiple modes of liver INR implementation have been proposed, however, they are also fraught with logistic challenges.22,35,36 Since the establishment of ISIliver requires calibration against the WHO IRP using manual tilt-tube methodology as well as 60 patient plasma samples spanning the spectrum of liver disease severity, it is impractical for most local laboratories to carry out this demanding task. Therefore, it was proposed that the calibration could be performed by the thromboplastin manufacturers. In this case, the packet inserts should include two ISIs, the ISIVKA and the ISIliver, and the coagulometer instrument(s) for which they were validated. Although possible, this proposal adds expenses and effort for the manufacturers and will take time before it can be fully endorsed and implemented. Furthermore, the concurrent existence of two similar INR systems will undoubtedly create confusion in the coagulation diagnostic testing laboratories and for the ordering physicians. Bellest et al advocated the use of standard pools of plasma derived from patients with liver disease that individual Seminars in Thrombosis & Hemostasis
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laboratories could use to derive the ISIliver, similar to the certified plasmas in the local verification/calibration of ISIVKA.31 However, large amounts of plasma would have to be collected, processed, and coordinated for distribution to individual coagulation laboratories. Frozen plasma is not ideal as a long-term reference material due to stability issues and lyophilized plasma although stable may alter the functionality of some coagulation factors. Another option is to have a centralized approach with only a few reference laboratories performing the liver INR testing for patients on the liver transplant waiting list, optimally using the same thromboplastin/instrument combination and establishing the ISIliver following the calibration procedure described by the two seminal papers described earlier. Ideally, serum testing of bilirubin and creatinine should also be performed in the same reference laboratory, since these two assays are also not devoid of laboratory variations. However, such centralization would be difficult to implement in many areas and countries and may be impractical for very sick patients whose MELD score needs to be recertified on a daily basis.
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Another way, as suggested by the Scientific and Standardization Committee (SSC) on anticoagulation control of the International Society of Thrombosis and Hemostasis (ISTH),37 is to choose reagents that give similar ISI values (CV < 10%) irrespective of whether such calibration is derived from the plasma samples of patients on VKA therapy or those with liver disease. From the Tripodi et al and Bellest et al studies as well as Burroughs et al’s research, it is apparent that certain reagents are equally sensitive to both VKA therapy and liver disease plasmas.30,31,33 Two promising candidates are the human recombinant thromboplastins, Innovin and RecombiPlasTin. Innovin is the thromboplastin used in the original Mayo Clinic derivation and validation of MELD score in endstage liver disease patients, and the most studied. RecombiPlasTin was tested side by side with Innovin in the Tripodi et al study30 and showed a negligible difference from Innovin in terms of INRVKA and MELDVKA score, as well as the standardized INRliver and MELDliver score. In such scenarios, it seems that one reagent with one assigned ISI can be utilized for testing plasmas from both types of patients. This might be an ideal solution since the same ISI would then be used for both indications, and obviates the need to set up a new “INR.” A logistic concern for this option is that one cannot compel laboratories to purchase a particular reagent. In addition, further validation is also required. Removing INR from the MELD score calculation has also been suggested, and preliminary results support the validity of this approach.38 However, the study population was limited to hepatitis C and/or alcoholic cirrhosis patients, and the impact of omitting the INR in the MELD score calculation in liver disease patients with other etiologies such as biliary cirrhosis, autoimmune hepatitis or other metabolic disorders was not assessed. There are also proposals for replacing INR with other parameters of hepatic synthetic reserve, such as measurements of plasma coagulation factors V or VII.39 However, such factor assays are relatively expensive, not routinely tested in patients with liver disease, and generally lack standardization. Recently, a revised MELD score found that capping the INR value at 3 led to the improved waitlist mortality prediction, and theoretically this INR cap could reduce some of the impact of the variability in INRvka on the calculated MELD score.40 Finally, the Quick PT is not the only kind of PT test available. Although less commonly used, the Owren PT system offers an alternative. The Owren PT reagents are supplemented with bovine factor V and fibrinogen, two clotting factors that are not affected by VKAs but which are variably affected in liver disease patients.41 Tripodi et al had included one Owren reagent, Thrombotest, in their study and observed minimal difference between the ISIliver and ISIVKA from this thromboplastin. The two ISI values were also very close to that of the RecombiPlasTin.30 Magnusson et al went a step further and studied the Owren method in a multilaboratory nationwide study.42 They showed that using three Owren reagents, namely, SPA þ, Owren PT or the Nycotest PT, there was no difference in the ISI whether it was derived from patients on VKA therapy or from patients with liver disease. However, its appropriateness to be used in
Gatt et al.
the MELD score calculation and subsequent liver transplant prioritization remains controversial, as the deficiency of factor V and fibrinogen associated with hepatic synthetic dysfunction would be masked by the system.43 Factor V is believed to be a reliable marker of severity of liver disease and plasma levels tend to decrease with worsening liver failure. Fibrinogen plasma levels also decrease, especially with very severe liver disease, albeit to a lesser extent than factor V. Although in the Tripodi et al study the Thrombotest was as sensitive to liver disease-induced coagulopathy as was RecombiPlasTin, this study only included 57 liver cirrhosis patients with 18 of them classified as CTP class C, the class with the most severe liver disease in the three-tier grading system. The validity of the above findings and the use of Owren reagent in the MELD score calculation would require further evaluation in larger cohorts of chronic liver disease patients, especially large population of patients with more severe liver disease. Future efforts for optimizing the INR in patients with liver disease would require testing in large cohorts of waitlisted patients to demonstrate validated model calibration and discrimination using INRliver as seen in newer iterations of the MELD model.40,44 It is not practical to expect local coagulation laboratories to perform such clinical validation. Such validation studies could be done by selecting laboratories and medical centers with extensive expertise in coagulation laboratory testing and liver transplantation. Information from these studies should be documented and shared publicly.
Conclusions In 2007 an alternative INR system was described for patients with liver disease to standardize the reporting of INR in this patient population. This system minimizes the interlaboratory variations in the MELD score and ensures organ allocations for liver transplantation to the sickest patients. However, the introduction of the INRliver has been hampered with multiple practical and logistical issues and as yet is not being widely adopted. As Marlar predicted soon after its description, both laboratorians and clinicians would have to work together if there was going to be any change toward liver INR.36 The SSC on anticoagulation control of the ISTH has made recommendations for potential use of INRliver testing and reporting.37 It was recommended that the INRVKA should not be used interchangeably for patients with coagulopathy of liver disease. For laboratories using a particular thromboplastin reagent that has been proven to yield similar ISIs whether for patients on VKA therapy or those with liver disease one thromboplastin and ISI can be used for both indications. In general, thromboplastins that have consistently demonstrated close similarity between the ISIliver and ISIVKA should be the preferred thromboplastins used for measurement of INR for calculation of MELD scores. Importantly, although the INRliver has been demonstrated to result in reduced variability of MELD scores, compared with the INRVKA, its effect on mortality prediction in patients with liver disease remains to be demonstrated. Seminars in Thrombosis & Hemostasis
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From Vitamin K Antagonists to Liver INR
From Vitamin K Antagonists to Liver INR
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References 1 Schofield FW. Damaged sweet clover: the cause of a new
2
3
4 5 6 7
8
9
10 11 12
13
14
15
16
17
18
19
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