Nikhil Chauhan

European Concerted Action on Anticoagulation

International sensitivity index (ISI) calibration of whole blood prothrombin time (PT) monitors is too complex. We previously simplified the method by using European Concerted Action on Anticoagulation (ECAA) lyophilized plasma samples with the TAS PT-NC (Bayer AG, Leverkusen, Germany) and the CoaguChek Mini (Roche Diagnostics, Mannheim, Germany) whole blood PT monitoring systems. The TAS PT-NC required a correction derived from the line of equivalence. Monte Carlo bootstrap analysis of reducing numbers of test samples was performed with both systems. Plasma samples from patients receiving coumarin (coumarin samples), healthy subjects (normal samples), and plasma samples artificially depleted of coagulation factors were used. With the TAS PT-NC, 20 coumarin samples or 20 artificially depleted samples with 7 normal samples gave reliable ISI and international normalized ratio and satisfactory precision. With the CoaguChek Mini, 30 coumarin and 10 normal samples were required. Simplification of ISI calibration of the 2 monitoring systems is possible using fewer ECAA lyophilized plasma samples than the 80 required according to the World Health Organization guidelines for conventional PT systems and previously recommended for fresh plasma samples tested on the same 2 monitoring systems.

Reliability of international normalised ratios from two point of care test systems: comparison with conventional methods

Abstract Design Comparison of the INRs from the two systems with a “true” INR on a conventional manual test from the same sample of blood. Setting 10 European Concerted Action on Anticoagulation centres. Participants 600 patients on long term dosage of warfarin. Main outcome measures Comparable results between the different methods. Results The mean displayed INR differed by 21.3% between the two point of care test monitoring systems. The INR on one system was 15.2% higher, on average, than the true INR, but on the other system the INR was 7.1% lower. The percentage difference between the mean displayed INR and the true INR at individual centres varied considerably with both systems. Conclusions Improved international sensitivity index calibration of point of care test monitors by their manufacturers is needed, and better methods of quality control of individual instruments by their users are also needed.

European Concerted Action on Anticoagulation (ECAA): multicentre international sensitivity index calibration of two types of point-of-care prothrombin time monitor systems

Summary. A multicentre modified World Health Organization (WHO)‐type international sensitivity index (ISI) calibration has been performed at 10 European Concerted Action on Anticoagulation (ECAA) national laboratories using non‐citrated whole‐blood on two point‐of‐care test (POCT) prothrombin time (PT) monitor systems, CoaguChek Mini and TAS PT‐NC, using single lots of test cards/strips. The relevant species (human and rabbit) WHO international reference preparations (IRPs) were tested with the manual PT technique on citrated plasma from the same blood donations. The ISI was calculated from the slope of the orthogonal regression line relating log PT (POCT) to log PT (IRP). The mean ISI of the CoaguChek Mini system was 1·75 and 1·13 with the prothrombin time non‐citrated Thrombolytic Assessment System (TAS PT‐NC). With the CoaguChek Mini system, seven out of 10 calibrations exceeded the current 3% WHO recommended limit for the coefficient of variation (CV) of the slope with conventional PT testing, whereas with the TAS PT‐NC system, it was eight out of 10. All the POCT calibrations had a CV of the slope < 5%. It is suggested that this level of precision be adopted as the limit of acceptability of calibration of these monitor systems. In these circumstances, the modified WHO‐type ISI calibration appeared to be satisfactory for the POCT whole‐blood monitors.

European Concerted Action on Anticoagulation (ECAA). An assessment of lyophilised plasmas for ISI calibration of CoaguChek and TAS whole blood prothrombin time monitors

Aims: The recommended method for the international sensitivity index (ISI) calibration of whole blood point of care testing (POCT) prothrombin time (PT) systems was originally described by Tripodi et al in 1993 but is too complex and demanding. The present European Concerted Action on Anticoagulation (ECAA) study aimed to assess the reliability of simpler ISI calibration using lyophilised plasma samples. Methods: ISI calibrations using three different types of ECAA lyophilised plasma samples (artificially depleted, individual, and pooled coumarin) were compared with whole blood calibrations on CoaguChek Mini and TAS PT-NC POCT monitors at 10 centres. Results: With CoaguChek Mini systems, lyophilised coumarin plasma samples (both single donation and pooled) gave ISI and international normalised ratio (INR) values comparable to whole blood. With artificially depleted plasma, ISI and INR values were too high. With TAS PT-NC systems, all three types of lyophilised plasma samples gave inaccurate ISI and unreliable INR results, similar to previous ECAA findings with fresh plasma calibrations. Conclusions: With CoaguChek Mini systems, ISI calibration can be simplified by the use of ECAA lyophilised plasma samples from coumarin treated patients. Further study is needed to devise a simpler calibration method for the TAS PT-NC system.

European Concerted Action on Anticoagulation. A multicentre calibration study of WHO international reference preparations for thromboplastin, rabbit (RBT/90) and human (rTF/95)

A 10 centre calibration was performed after six years to determine the international sensitivity index (ISI) of rTF/95 relative to RBT/90, and to assess any international normalised ratio (INR) bias compared with the original multicentre calibration. After exclusion of one outlying centre, the follow up calibration gave a mean ISI for rTF/95 of 0.99, which although a small difference, is significantly greater than the mean ISI of 0.94 obtained previously. The change in ISI for international reference preparation (IRP) rTF/95 relative to RBT/90 would lead to a slight bias in INR for human compared with rabbit thromboplastins. At a theoretical INR of 3.0, the INR bias is 6.0%, and this is below the accepted 10% level of clinical relevance. Ongoing stability monitoring of World Health Organisation thromboplastin IRP is advised.

European Concerted Action on Anticoagulation (ECAA): an assessment of a method for ISI calibration of two whole blood point-of-care PT monitor systems based on lyophilized plasmas using whole blood equivalent PT.

Summary.  Previously, the attempt to simplify the International Sensitivity Index (ISI) calibration of the CoaguChek Mini whole blood point‐of‐care test prothrombin time (PT) monitor system was successful using lyophilized plasmas from coumarin‐treated patients but not with lyophilized artificially depleted plasmas. With the TAS PT‐NC monitor system, both types of plasma failed to provide reliable calibrations. The present study assesses a procedure for the ISI calibration of a TAS PT‐NC and CoaguChek Mini whole blood point‐of‐care test PT monitor systems using lyophilized plasmas. Using lyophilized artificially depleted and coumarin plasma calibrations, we have evaluated a correction for the monitor displayed PT. This was based on a ‘line of equivalence’ derived from the relationship between whole blood and fresh plasma PT with both types of monitor system. With the TAS PT‐NC, the use of this ‘line of equivalence’ resulted in reliable ISI with both lyophilized coumarin and artificially depleted plasmas. There was no significant difference between mean monitor and mean reference International Normalized Ratio (INR) with the artificially depleted plasmas. With the lyophilized coumarin plasma calibrations there was only a small INR difference. Correction with the ‘line of equivalence’ therefore facilitates calibration of the TAS PT‐NC with lyophilized plasmas. With the CoaguChek Mini, the correction based on the ‘line of equivalence’ did not improve results but was not required with this system.

European Concerted Action on Anticoagulation. Correction of displayed international normalized ratio on two point‐of‐care test whole‐blood prothrombin time monitors (CoaguChek Mini and TAS PT‐NC) by independent international sensitivity index calibration

Summary. The international normalized ratio (INR) on two widely used point‐of‐care test (POCT) prothrombin time (PT) monitors (CoaguChek Mini and TAS PT‐NC) differed considerably and also differed from the ‘true’ INR obtained on the same samples using a manual PT and the same species thromboplastin international reference preparation. Agreement between the displayed INR and difference from ‘true’ INR has been reassessed following an independent international sensitivity index (ISI) calibration of the two systems. The displayed INRs taken at seven centres were compared with ‘true’ INRs from the same blood donations and INRs based on the resulting ISI. The overall difference between the displayed INRs on the two monitor systems was reduced from 21·0% to 3·5%. The overall difference in mean INR of system A from the ‘true’ INR was reduced from 19·0% to 9·5% and of system B from 6·8% to 0·3%, but individual centres results still showed considerable mean INR variability. Differences between overall displayed INR with the two monitor systems have been reduced by an independent multicentre calibration, and agreement with ‘true’ INR on the same blood samples improved. However, marked variability in mean INR at individual centres remained after ISI correction, which demonstrates the need for external quality control of individual POCT whole‐blood PT monitors.

European concerted action on anticoagulation: Minimum numbers of lyophilized plasma samples for ISI calibration of CoaguChek and TAS point-of-care whole blood prothrombin time monitors

International sensitivity index (ISI) calibration of whole blood prothrombin time (PT) monitors is too complex. We previously simplified the method by using European Concerted Action on Anticoagulation (ECAA) lyophilized plasma samples with the TAS PT-NC (Bayer AG, Leverkusen, Germany) and the CoaguChek Mini (Roche Diagnostics, Mannheim, Germany) whole blood PT monitoring systems. The TAS PT-NC required a correction derived from the line of equivalence. Monte Carlo bootstrap analysis of reducing numbers of test samples was performed with both systems. Plasma samples from patients receiving coumarin (coumarin samples), healthy subjects (normal samples), and plasma samples artificially depleted of coagulation factors were used. With the TAS PT-NC, 20 coumarin samples or 20 artificially depleted samples with 7 normal samples gave reliable ISI and international normalized ratio and satisfactory precision. With the CoaguChek Mini, 30 coumarin and 10 normal samples were required. Simplification of ISI calibration of the 2 monitoring systems is possible using fewer ECAA lyophilized plasma samples than the 80 required according to the World Health Organization guidelines for conventional PT systems and previously recommended for fresh plasma samples tested on the same 2 monitoring systems.

European Concerted Action on Anticoagulation (ECAA). Minimum number of centres for reliable International Sensitivity Index calibration of CoaguChek and TAS point-of-care whole blood monitors

INTRODUCTION Prothrombin time (PT) test systems require multicentre calibration for reliable International Sensitivity Index (ISI). Multicentre calibration of CoaguChek Mini and TAS PT-NC point-of-care test (POCT) systems is less precise than conventional PT testing. The aim of the present study was to determine the number of centres required to give reliable ISI and International Normalised Ratio (INR) with these two POCT whole blood PT monitors. MATERIALS AND METHODS A simulation study, based on results of a 10-centre calibration exercise, was performed to assess reliability of ISI and INR when the number of centres was reduced from 10 to 2. RESULTS AND CONCLUSIONS With both systems, the range of ISI and INR deviation increased as the number of centres was reduced. For the CoaguChek Mini, at least five centres were needed for satisfactory INR deviation in 95% of calibrations. With the TAS PT-NC, three centres gave satisfactory INR at this level. The number of centres required for multicentre calibration of these two POCT PT systems is greater than the two proposed by World Health Organisation (WHO) Guidelines for conventional PT testing.