Defining a metrologically traceable and sustainable calibration hierarchy of international normalized ratio for monitoring of vitamin K antagonist treatment in accordance with International Organization for Standardization (ISO) 17511:2020 standard: communication from the International Federation of Clinical Chemistry and Laboratory Medicine-SSC/ISTH working group on prothrombin time/international normalized ratio standardization

Calibration of prothrombin time (PT) in terms of international normalized ratio (INR) has been outlined in "Guidelines for thromboplastins and plasmas used to control oral anticoagulant therapy" (World Health Organization, 2013). The international standard ISO 17511:2020 presents requirements for manufacturers of in vitro diagnostic (IVD) medical devices (MDs) for documenting the calibration hierarchy for a measured quantity in human samples using a specified IVD MD. The objective of this article is to define an unequivocal, metrologically traceable calibration hierarchy for the INR measured in plasma as well as in whole blood samples. Calibration of PT and INR for IVD MDs according to World Health Organization guidelines is similar to that in cases where there is a reference measurement procedure that defines the measurand for value assignment as described in ISO 17511:2020. We conclude that, for PT/INR standardization, the optimal calibration hierarchy includes a primary process to prepare an international reference reagent and measurement procedure that defines the measurand by a value assignment protocol conforming to clause 5.3 of ISO 17511:2020. A panel of freshly prepared human plasma samples from healthy adult individuals and patients on vitamin K antagonists is used as a commutable secondary calibrator as described in ISO 17511:2020. A sustainable metrologically traceable calibration hierarchy for INR should be based on an international protocol for value assignment with a single primary reference thromboplastin and the harmonized manual tilt tube technique for clotting time determination. The primary international reference thromboplastin reagent should be used only for calibration of successive batches of the secondary reference thromboplastin reagent.

Therapeutic quality control in a regional thrombosis center: The effect of changing the target intensity of anticoagulation with vitamin K antagonists

BACKGROUND

The target ranges (TR) for anticoagulation with vitamin K antagonists (VKA) in the Netherlands were changed in 2016 from INR 2.0-3.5 ('low intensity') and INR 2.5-4.0 ('high intensity') to INR 2.0-3.0 and INR 2.5-3.5, respectively.

AIM

To assess the effect of the TR change on therapeutic quality control (TQC) in a Dutch regional thrombosis center taking care of approximately 3600-5500 patients annually.

METHODS

TQC of chronically treated patients was assessed as the average time in therapeutic range (TTR). Evaluations were performed for non-self-management (NSM), as well as self-management patients. INR percentiles were assessed from all INR determinations in all patients, i.e. including those of induction episodes and patients treated for a short-term.

RESULTS

The number of NSM patients treated chronically decreased gradually, while their average age increased, with a marginal but significant gradual increase in bleeding complications. In the period 2011-2015, i.e. before the TR change, there was a gradual increase of the TTR in NSM patients from 77.5% to 88.9% (low intensity) and from 75.3% to 84.1% (high intensity). In the same period, the median INR of all patients in the low and high intensity ranges decreased from 2.9 to 2.7, and from 3.3 to 3.2, respectively. The TTR in self-management patients remained virtually constant. After TR changes from 2016 on, the TTR of all NSM patients in the low and high intensity groups decreased to 77% and 70%, respectively, and median INRs decreased to 2.6 and 3.0, respectively.

CONCLUSIONS

Introduction of internationally harmonized target ranges in 2016 resulted in further lowering of median INR values in both target ranges. As expected, TTR was reduced slightly. These findings, together with a slight increase in average age and concomitant bleeding complications, suggest that the patients on long-term VKA treatment will require intensified monitoring and treatment.

Bias and uncertainty of the International Normalized Ratio determined with a whole blood point-of-care prothrombin time test device by comparison to a new International Standard for thromboplastin

BACKGROUND

Whole blood point-of-care PT/INR test devices, e.g. CoaguChek XS, are calibrated by their manufacturers. In the Netherlands, each new lot of test strips for CoaguChek XS is validated by a group of anticoagulant clinics collaborating with a Coagulation Reference Laboratory. In 2017, a new International Standard for recombinant human thromboplastin (coded rTF/16) has been established by the World Health Organization.

AIM

To assess uncertainty of the validation procedure and the magnitude of the INR bias of a series of consecutive lots of test strips imported in the Netherlands.

METHODS

CoaguChek XS test strip INR results were compared to INRs determined with the new International Standard rTF/16. Comparisons were made with variable numbers of blood samples obtained from patients treated with vitamin K-antagonists. Relationships between CoaguChek XS and rTF/16 results were determined with orthogonal regression analysis. The relationships were used to assess bias and uncertainty of bias.

RESULTS

Average bias between CoaguChek XS test results and rTF/16 depends on the INR level. Overall, there was a trend of increasing bias and increasing uncertainty with increasing INR values. Along the sequence of 47 consecutive lots, a temporary fluctuation of bias was observed. At an INR level of 3.0 the average bias was less than 10% in all cases, but at an INR of 4.0 there were 5 lots with average bias between 10 and 15%.

CONCLUSION

Validation of test strips is useful to assess bias but depends on availability of fresh patients' samples and traceability to an accepted Reference Measurement System.

Assignment of international normalized ratio to frozen and freeze-dried pooled plasmas

Objectives Frozen and freeze-dried plasmas may be used for local prothrombin time system calibration, for direct international normalized ratio (INR) determination, and for quality assessment. The purpose of the present study was to evaluate the usefulness of INRs assigned with various types of thromboplastins to frozen and freeze-dried pooled plasmas obtained from patients treated with vitamin K antagonists. Methods INRs were calculated according to the international sensitivity index (ISI) model using various thromboplastins and instruments, i.e. International Standards for thromboplastin as well as six commercial reagents prepared from rabbit and bovine brain, and recombinant human tissue factor. The uncertainty of the INRs was assessed using the standard deviations of clotting times and ISI values. Commutability of the plasmas was assessed according to the approved Clinical and Laboratory Standards Institute (CLSI) Guideline EP30-A. Validation of a set of six frozen plasma pools for direct INR determination was performed according to the Subcommittee on Control of Anticoagulation of the Scientific and Standardization Committee of the International Society on Thrombosis and Haemostasis (SSC/ISTH) guidelines. Results For all frozen and freeze-dried plasmas, the INRs calculated with bovine thromboplastin Thrombotest were lower than the INRs assigned with other thromboplastins. With a few exceptions, the frozen and freeze-dried pooled plasmas were commutable. When the set of six frozen plasma pools was used for local calibration, the analytical bias of the INR was less than ±10% for all commercial reagents except Thrombotest. Conclusions Processing of fresh plasmas to prepare pooled frozen plasmas and freeze-dried plasmas may lead to different INR assignments depending on the thromboplastin used. Despite minor INR differences, a set of six frozen plasma pools could be used for local calibration by direct INR determination.

Accuracy assessment of consecutive test strip lots for whole blood INR point-of-care instruments: clarifying the role of frozen plasma pools

Background In the Netherlands, each new lot of test strips for the CoaguChek XS is validated by a group of collaborating centers. The purpose of this study was to assess the accuracy of the international normalized ratio (INR) measured with consecutive test strip lots and the suitability of frozen plasma pools for accuracy evaluation. Methods Each year, a particular lot of CoaguChek XS test strips is used as reference lot. The reference lots have been validated with the International Standard for thromboplastin rTF/09, yielding a mathematical relationship (R1) between reference lot INR and International Standard INR. New lots are compared to the reference lot using patients' capillary blood samples, yielding a relationship (R2) between the new lot INR and the reference lot INR. INRs of the blood samples were within the 1.5-4.5 interval. In parallel, three frozen plasmas pools are analyzed with the test strips. The distance of each plasma point to the line of relationship R2 was assessed. Results Fifty-four test strip lots have been evaluated during 3 years (2014-2016). Mean INR differences between test strip lot and International Standard rTF/09 varied between -0.14 and +0.20 (-4% and +8%, respectively). A positive trend with strip lot sequence number was observed (p<0.001). In several cases, the distance of the frozen plasmas to the whole blood relationship (R2) was greater than the critical value for commutability. Conclusions Using whole blood, all evaluated test strip lots met the analytical bias criterion of ±10%. Frozen plasma pools behave differently compared to whole blood and are not suitable for assessing absolute accuracy of new CoaguChek XS test strips.

Paving the way for establishing a reference measurement system for standardization of plasma prothrombin time: Harmonizing the manual tilt tube method

BACKGROUND

International normalized ratio (INR) is traceable to World Health Organization (WHO) International Standards for thromboplastins. International Standards must be used with a manual tilt tube technique (MTT) for prothrombin time (PT) determination. An important part of the total variability of INR is due to poor harmonization of MTT across WHO reference laboratories.

OBJECTIVES

To determine the origins of PT differences between operators performing MTT and to develop a harmonized MTT.

METHODS

Two workshops were held where WHO reference laboratory operators could compare their PTs using MTT and the same equipment. A harmonized MTT was used by seven operators in the second workshop.

RESULTS

Differences have been observed in tilting frequency and in the height of pipetting plasma in the test tube. At the beginning of the first workshop, the tilting cycle time varied between 1.1 and 2.7 seconds. The mean PT of normal plasma obtained by pipetting plasma at the top of the tube was 14.3 seconds but was 12.9 seconds when plasma was pipetted at the bottom of the tube. When using the harmonized MTT for WHO International Standard rTF/16, the differences between operators were not greater than 1.1 seconds in normal plasma, and not greater than 1.3 seconds in patient plasma with average INR of 3.0. INR between-operator coefficient of variation was 2.3%.

CONCLUSION

Application of a harmonized MTT in three reference laboratories resulted in substantial reduction of between-operator variation of PT and INR. The harmonized MTT is proposed as Candidate Reference Measurement Procedure.

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

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.

The Effect of Some Instruments for Prothrombin Time Testing on the International Sensitivity Index (ISI) of Two Rabbit Tissue Thromboplastin Reagents

Two commercial rabbit tissue thromboplastins were calibrated against the International Reference Preparation for rabbit thromboplastin (coded RBT/79) by one laboratory using the manual technique, a semi-automatic electro-mechanical coagulometer and three different automatic photo-optical instruments. The calibration of the two reagents was performed in three and two different exercises, respectively, and showed good reproducibility of the procedure. The purpose of calibration is providing a formula for the assessment of the International Normalized Ratio (INR) for patients receiving oral anticoagulants. The World Health Organization (WHO) model for thromboplastin calibration leads to the equation INR = RISI, in which R is the prothrombin time ratio and ISI the International Sensitivity Index of the calibrated thromboplastin/instrument system. This equation was adequate for one reagent, but not for the other when it was used in combination with the four instruments. At therapeutic intensities of anticoagulation, the deviation from the WHO-model observed for the second reagent was clinically insignificant. The WHO model was fully adequate when the second reagent was used with the manual technique. For both thromboplastins, there were statistically significant differences in ISI between the four instruments. The largest difference-amounting to approximately 10%-was observed between two photo-optical instruments. The order of instruments with increasing ISI was the same for the two reagents. It is recommended that thromboplastin manufacturers specify the instruments used for calibration of their reagent.