Local International Normalized Ratio Research Articles

International normalized ratio (INR) testing in Europe: between-laboratory comparability of test results obtained by Quick and Owren reagents.

Abstract Background: This study was designed to obtain an overview of the analytical quality of the prothrombin time, reported as international normalized ratio (INR) and to assess the variation of INR results between European laboratories, the difference between Quick-type and Owren-type methods and the effect of using local INR calibration or not. In addition, we assessed the variation in INR results obtained for a single donation in comparison with a pool of several plasmas. Methods: A set of four different lyophilized plasma samples were distributed via national EQA organizations to participating laboratories for INR measurement. Results: Between-laboratory variation was lower in the Owren group than in the Quick group (on average: 6.7% vs. 8.1%, respectively). Differences in the mean INR value between the Owren and Quick group were relatively small (<0.20 INR). Between-laboratory variation was lower after local INR calibration (CV: 6.7% vs. 8.6%). For laboratories performing local calibration, the between-laboratory variation was quite similar for the Owren and Quick group (on average: 6.5% and 6.7%, respectively). Clinically significant differences in INR results (difference in INR>0.5) were observed between different reagents. No systematic significant differences in the between-laboratory variation for a single-plasma sample and a pooled plasma sample were observed. Conclusions: The comparability for laboratories using local calibration of their thromboplastin reagent is better than for laboratories not performing local calibration. Implementing local calibration is strongly recommended for the measurement of INR.

Monitoring Coumadin-The Original Oral Anticoagulant

1. David L. McGlasson, MS, MLS(ASCP)CM[⇑][1] 1. 59th Clinical Research Division Laboratory Services, Wilford Hall Ambulatory Service Center, Lackland AFB, TX 1. Address for Correspondence- David L. McGlasson MS, MLS(ASCP)CM, Clinical Research Scientist, 59th Clinical Research Division Laboratory Services, Wilford Hall Ambulatory Service Center, Lackland AFB, TX 78236-9908, 210-292-6555, David.mcglasson{at}us.af.mil 1. Discuss the use of the prothrombin time (PT)/international normalized ratio (INR) for monitoring subjects on the oral anticoagulant Coumadin. 2. Be aware of the relevance of clinical and genetic data (CYP2C9, VKORC1) in predicting Coumadin dosing in subjects requiring different INR ranges. 3. Be knowledgeable of the chromogenic factor X (CFX) assay for monitoring Coumadin in subjects with lupus anticoagulants, transitioning subjects to Coumadin from direct thrombin inhibitors and for accurately following patients with elevated INRs. The prothrombin time (PT) has been the primary screening test for the laboratory evaluation of patients with an acquired or inherited coagulation factor deficiency of what was originally known as the extrinsic or common pathway of coagulation. It is used in conjunction with the international normalized ratio (INR) to monitor oral anticoagulant therapy (OAT) subjects on the oral anticoagulant, Coumadin.1 Tissue factor (TF) in vivo activates the coagulation cascade through the formation of the TF/FVIIa complex. The PT adds a TF/calcium mixture to citrated platelet-poor-plasma (PPP) in vitro to obtain the time for clot formation. There are many variables such as proper specimen collection, multiple reagent/instrument combinations and, most importantly, the sources of thromboplastin, usually rabbit and recombinant tissue factor, selected for the assay.1,2 The INR and Local INR Calibration The commercial thromboplastins vary widely in their sensitivities to Coumadin. Instrumentation differences (photo-optic or mechanical clot detection) may also play a clinically significant role in the performance of the PT/INR assay. With so many variables, providers treating subjects on OAT can be confused when comparing patient results from a number of laboratories using different reagent/instrument combinations. A facility that uses a high sensitivity thromboplastin will generate longer PT results than an assay that uses a low sensitivity thromboplastin. Therefore a subject on OAT may have a PT of 14 seconds with a low sensitivity reagent or 18 seconds with a more sensitive thromboplastin. Hence a subject monitored with insensitive thromboplastins would appear to require a higher dosage of Coumadin to… ABBREVIATIONS: CFX - chromogenic factor X; CLSI - Clinical and Laboratory Standards Institute; CYP2C9 - cytochrome P450; DTI - direct thrombin inhibitor; FII - coagulation factor II (prothrombin); FX - coagulation factor X; GMNPT - geometric mean normal prothrombin time; INR - international normalized ratio; IRP - international reference preparation; ISI - international sensitivity index; LA - lupus anticoagulant; OAT - oral anticoagulation therapy; PGX - pharmacogenetics; PST - patient self-testing; PT - prothrombin time; TTR - time in therapeutic range; VKORC1 - vitamin K epoxide reductase complex subunit 1; WHO - World Health Organization 1. Discuss the use of the prothrombin time (PT)/international normalized ratio (INR) for monitoring subjects on the oral anticoagulant Coumadin. 2. Be aware of the relevance of clinical and genetic data (CYP2C9, VKORC1) in predicting Coumadin dosing in subjects requiring different INR ranges. 3. Be knowledgeable of the chromogenic factor X (CFX) assay for monitoring Coumadin in subjects with lupus anticoagulants, transitioning subjects to Coumadin from direct thrombin inhibitors and for accurately following patients with elevated INRs. [1]: #corresp-1

Coagulometer international sensitivity index (ISI) derivation, a rapid method using the prothrombin time/international normalized ratio (PT/INR) Line: a multicenter study

The original WHO procedure for prothrombin time (PT) standardization has been almost entirely abandoned because of the universal use of PT coagulometers. These often give different international normalized ratio (INR) results from the manual method, between individual makes of instruments and with instruments from the same manufacture. A simple procedure is required to derive local INR with coagulometers. The PT/INR Line method has recently been developed using five European Concerted Action on Anticoagulation (ECAA) certified plasmas to derive local INR. This procedure has been modified to derive a coagulometer PT/INR Line providing International Sensitivity Index (ISI) and mean normal PT (MNPT) for coagulometers and give local INR. Results have been compared with conventional ISI calibrations at the same laboratories. With human thromboplastins, mean ISI by local calibration was 0.93 (range: 0.77-1.16). With the PT/INR Line, mean coagulometer ISI was higher, for example 0.99 (0.84-1.23) but using the PT/INR Line derived MNPT there was no difference in local INR. Between-centre INR variation of a certified validation plasma was reduced with human and bovine reagents after correction with local ISI calibrations and the PT/INR Line. The PT/INR Line-ISI with its derived MNPT is shown to provide reliable local INR with the 13 different reagent/coagulometer combinations at the 28 centres in this international study.

The prothrombin time/international normalized ratio (PT/INR) Line: derivation of local INR with commercial thromboplastins and coagulometers – two independent studies

The WHO scheme for prothrombin time (PT) standardization has been limited in application, because of its difficulties in implementation, particularly the need for mandatory manual PT testing and for local provision of thromboplastin international reference preparations (IRP). The value of a new simpler procedure to derive international normalized ratio (INR), the PT/INR Line, based on only five European Concerted Action on Anticoagulation (ECAA) calibrant plasmas certified by experienced centres has been assessed in two independent exercises using a range of commercial thromboplastins and coagulometers. INRs were compared with manual certified values with thromboplastin IRP from expert centres and in the second study also with INRs from local ISI calibrations. In the first study with the PT/INR Line, 8.7% deviation from certified INRs was reduced to 1.1% with human reagents, and from 7.0% to 2.6% with rabbit reagents. In the second study, deviation was reduced from 11.2% to 0.4% with human reagents by both local ISI calibration and the PT/INR Line. With rabbit reagents, 10.4% deviation was reduced to 1.1% with both procedures; 4.9% deviation was reduced to 0.5% with bovine/combined reagents with local ISI calibrations and to 2.9% with the PT/INR Line. Mean INR dispersion was reduced with all thromboplastins and automated systems using the PT/INR Line. The procedure using the PT/INR Line provides reliable INR derivation without the need for WHO ISI calibration across the range of locally used commercial thromboplastins and automated PT systems included in two independent international studies.

Usefulness of lyophilized calibration plasmas for International Normalized Ratio determination with the bovine combined thromboplastin (Thrombotest): results of a collaborative study

The logical solution to account for the influence of coagulometers on the International Sensitivity Index (ISI) is local calibration with freeze-dried plasmas. However, because of their unpredictable behavior these plasmas must be validated before large-scale implementation. We report on a collaborative exercise designed to evaluate the suitability of a set of such plasmas used with Thrombotest in combination with a coagulometer provided by the manufacturer to be used with that reagent. This was a two-step study. First, one lot of reagent was calibrated against the international standard OBT/79 in two expert laboratories. The calibrated lot was then used as an intermediate standard to calibrate two additional lots of the same reagent in four field laboratories where the ISI was determined for both plasma and native blood. The International Normalized Ratio (INR) for the patient plasmas tested in each laboratory were calculated using two algorithms: the World Health Organization-recommended ISI mode (gold standard), and the simplified calibration plasma mode. In the latter, the INR was derived from the local calibration curve constructed by plotting the certified INR versus local coagulation times obtained with calibration plasmas. The between-algorithm INR differences indicate that this set of calibration plasmas may be employed for local INR calibration of the investigated reagent/instrument combination, especially when plasma is used for INR determination where the average INR (range) difference is 5% (3-13%) or 2% (3-8%) according to whether the INRs to calibration plasmas were assigned by the manufacturer or by the two expert laboratories. A slight but measurable difference of the INR may be predicted [9% (6-20%) or 6% (8-15%)] if this set of calibration plasmas is used for local calibration when native blood is employed for INR determination. Whether this bias is of practical significance is to be determined.

Local INR calibration of the Owren type prothrombin assay greatly improves the intra- and interlaboratory variation

In 1999, a simplified procedure for calibration of the Owren prothrombin time (Owren PT) assay was introduced by a working group of the organisation for national quality assurance in laboratory medicine in Sweden. The new protocol allowed local calibration by means of only two lyophilised national plasma calibrators and expression of results as an international normalized ratio (INR). This is our report of a three-year follow-up involving the analysis of data from all laboratories, in hospitals (n=88 in 2002) and primary health care units (n=246 in 2002) that perform the Owren PT assay in Sweden. The interlaboratory variation was significantly improved after the introduction of the new calibration procedure. For the larger hospital-based laboratories, the mean coefficient of variation (CV) was reduced from 7.9% to 5.2% (p<0.0001) when analysing test materials with INR range 2-4. In the higher INR range (>4), the CV was reduced even further, from 10.4% to 6.8% (p<0.0001). The corresponding results from smaller laboratories in the primary health care units showed a similar decrease in CV from 8.2% to 5.7% in the INR range 2-4 (p<0.0001). At the INR range >4, the CV was reduced from 9.5% to 7.8%. The intralaboratory variation was also improved for both types of laboratory categories. This study shows an improved precision, with CV less than 6% at the therapeutic INR range, for both hospital-based laboratories and smaller laboratories in the primary health care system. The results indicate that the Owren PT assay is well suited for local INR calibration employing only two calibrant plasmas in a simplified procedure.

Use of Lyophilized Calibrant Plasmas for Simplified International Normalized Ratio Determination with a Human Tissue Factor Thromboplastin Reagent Derived from Cultured Human Cells

For monitoring of treatment with oral anticoagulants, the clotting time obtained in the prothrombin time (PT) test is transformed to the International Normalized Ratio (INR) with use of a system-specific International Sensitivity Index (ISI). The calibrant plasma procedure (CPP) is an alternative approach to INR calculation based on the use of a set of lyophilized plasmas with assigned INRs. With the CPP, a linear relationship is established between log(PT) and log(INR), using orthogonal regression. CPP was validated for Simplastin HTF, a new human tissue factor reagent derived from cultured human cells. CPP precision was assessed as the CV of the slope of the regression line. The accuracy of the CPP was determined by comparing the INR obtained with the CPP with that obtained with the established ISI-based reference method. INRs of the calibrants were assigned by different routes: by manufacturer (consensus labeling) or by use of Simplastin HTF or International Reference Preparations (IRPs; rTF/95 or RBT/90). The mean CV of the CPP regression slope ranged from 1.0% (Simplastin HTF reagent-specific INR) to 2.4% (INR assigned with rTF/95). INRs calculated with the CPP were similar to those obtained with the reference method, but when the routes for assigning INRs to the calibrant plasmas were compared, the mean difference in INR between CPP and the reference method was smaller with Simplastin HTF reagent-specific values. In several (but not all) cases, this difference was significant (P <0.05, t-test). CPP can be used for local INR determination, but better precision and accuracy are obtained with reagent-specific INRs compared with INR assignment by consensus labeling or IRP.

Accuracy and Precision of the CoaguChek S versus Laboratory INRs in a Clinic

BACKGROUND:The CoaguChek S is the next-generation coagulation monitor for measuring the international normalized ratio (INR) that replaces the CoaguChek device. Studies are lacking comparing the CoaguChek S with local laboratory INR assessment to ensure its accuracy and precision for monitoring patients on anticoagulation.OBJECTIVE:To evaluate accuracy, precision, and technical ease-of-use of the CoaguChek S compared with laboratory measurements.METHODS:Accuracy was evaluated in 101 patients by parallel assessment of INRs (CoaguChek S and laboratory); precision was evaluated in 31 patients using duplicate INRs from CoaguChek S and laboratory and from liquid quality controls. Accuracy was determined using orthogonal regression, Bland—Altman plot, and clinical applicability (INRs discrepant in categorization of INR goal and resulting in different therapeutic decisions). Precision was examined by comparing mean difference ± SD between repeated INRs from CoaguChek S and laboratory, coefficient of variation (CV), and coefficient of repeatability (CR). The influence of low and elevated INRs on accuracy and precision was also examined. To assess ease-of-use of the monitor, the number of technical errors was recorded.RESULTS:The CoaguChek S significantly correlated to laboratory measurement (r = 0.93); 16.7% of INRs resulted in discrepant categorization and 24.5% would have required a different therapeutic plan. The CV and CR compared well between CoaguChek S and laboratory (6% vs. 4.9%; 0.455 vs. 0.346, respectively). When subgroups of INR values <4.0 and <3.0 were evaluated, the precision improved with both methods. Precision, based on liquid quality controls, was good (CV 4.6% = low-level; 3.3% = high-level). The CoaguChek S was found to have an error rate of 1.8%.CONCLUSIONS:The CoaguChek S is an accurate and precise alternative to laboratory assessment of the INR at values <4.0; it is an efficient device with a low likelihood of errors during testing.

A simplified statistical method for local INR using linear regression. European Concerted Action on Anticoagulation.

A simplified method of International Normalized Ratio (INR) derivation using linear regression of certified INR plotted against local prothrombin time (PT) results has been compared with INR from conventional orthogonal regression. Linear regression assumes error only with the local PT results whereas orthogonal regression assumes error with both reference and local results. The reliability of local INR derivation using lyophilized plasmas has been assessed in a collaborative study. INR from conventional fresh plasma International Sensitivity Index (ISI) calibrations have been compared with INR from calibrations with two types of lyophilized plasma, artificially depleted and coumarin. Although calibration slopes differed with the two types of analysis and the different lyophilized plasmas, both gave reasonable approximations to fresh plasma ISI calibrations. With orthogonal regression the overall percentage INR deviation was 5.25% with the artificially depleted plasmas and 6.85% for the results with lyophilized coumarins. With the linear regression, deviation was 8.40% for the artificially depleted plasmas and 5.05% for coumarin-treated patients' lyophilised-plasma. The simpler regression method appears to be worthy of further study as the present report has demonstrated that if the calibrant plasmas are accurately certified with the thromboplastin International Reference Plasma (IRP) results approximate to the conventionally determined INR using the manual PT technique. Coagulometers require further assessment.