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Letters to the Editor |
1 European Action on Anticoagulation, Central Facility, University of Manchester, Manchester, United Kingdom
2 Department of Clinical Biochemistry, Ribe County Hospital in Esbjerg, Esbjerg, Denmark
3 Bianchi Bonomi Hemophilia and, Thrombosis Centre, University and Instituto di Ricovero e, Cura a Carattere Scientifico, Maggiore Hospital, Milan, Italy
aAddress correspondence to this author at: European Action on Anticoagulation, Central Facility, Faculty of Life Sciences, University of Manchester, 3.239 Stopford Bldg., Oxford Rd., Manchester M13 9PT, United Kingdom. Fax 44-161-275-5316; e-mail ecaa{at}manchester.ac.uk.
To the Editor:
The European Concerted Action on Anticoagulation (ECAA) includes the founder and former long-term organizer of the UK National External Quality Assessment Scheme (NEQAS) and the WHO International External Quality Assessment Scheme in blood coagulation as well as current European national external quality assessment (EQA) organizers.
We strongly endorse the 2 main points made by Kitchen and his UK NEQAS colleagues, that EQA for point-of-care international normalized ratio (INR) devices should be mandatory and caution is required when interpreting data from lyophilized plasma samples analyzed with whole blood prothrombin time (PT) monitors. During the past several years, the ECAA and its successor, European Action on Anticoagulation (EAA), have published data on both of these concerns in more than 60 reports, including 4 in Clinical Chemistry.
With regard to plasma/whole blood differences, the ECAA initially studied the reliability of use of plasma samples in calibrating the CoaguChek monitor (1) with an international sensitivity index (ISI). An optimum formulation of calcium chloride for recalcification of plasma on the CoaguChek and TAS monitors was developed for ISI calibrations and EQA. In full multicenter ISI calibrations at 10 centers, the plasma/whole blood ISI difference was thereby reduced to 1%–6% with various lots of CoaguChek test strips, but small differences between whole blood and plasma persisted. The ECAA studies also revealed previously unsuspected differences in mean ISI with different CoaguChek test strip lots (2). In full calibrations performed at 3 centers, 1 lot gave a 13% ISI difference (1.51) from the mean of 3 others (1.74). A similar interlot difference was detected with both whole blood and plasma. The ECAA EQA plasmas in our 2006 Netherlands national field study (3) also showed evidence of interlot differences. The relatively low detection rate of interlot problems reported by Kitchen and coworkers for the investigation of up to 70 different lots, of which only 4 showed >10% difference from the median INR, can be regarded only as evidence of the limitations of the UK NEQAS approach and the need for more specific EQA.
Kitchen and coworkers challenge our use of the whole blood ISI to certify the INR of ECAA EQA plasmas. The whole blood ISI was preferred because of the small but constant difference in ISI between plasma and whole blood that persisted even with our modified recalcification. Thus adoption of the whole blood INR certification seemed preferable because this method gives less deviation from the certified values without affecting the underlying principles of EQA, which are more concerned with variability than absolute truth.
The UK NEQAS is designed to cover the whole range of PT testing systems. Therefore it would be difficult to provide a similarly precise analysis of performance to that contained in the EC-approved Technology Implementation Plan designed specifically for the EQA of the CoaguChek. The ECAA Technology Implementation Plan specifies that users of point-of-care testing (POCT) monitors should test them with EQA plasmas at intervals of not >6 months (or whenever there is a change of the lot of test strips).
ECAA surveys showed that the POCT PT monitors are less precise than traditional methods and that a minimum of 5 INR-certified EQA plasmas tested on the same day was required in an exercise to provide a reliable EQA of CoaguChek monitors. Sets of 5 ECAA EQA plasmas are therefore provided to users with diluent and calcium chloride. A 15% or more deviation from certified INR on 1 or more test plasmas in the set of 5 is classified as "unsatisfactory performance" (4). Immediate EQA is thus provided for CoaguChek users.
Traditional UK NEQAS analysis is slower and different depending on deviation from the overall median of all participants in an exercise and taking weeks or months to provide the results for a user. The ECAA/ECAT Netherlands study in our 2006 report showed that the ECAA method of rapid, "on the spot" analysis by percentage deviation from certified INR values gave results similar to those of traditional UK NEQAS analysis, and that any differences in detection rate of unsatisfactory performance were not attributable to the method of statistical analysis (5).
The UK NEQAS has so far not provided the requisite 5 CoaguChek–dedicated test samples in a single exercise. Combining results of serial exercises performed over a period of months or years as detailed in the report by Kitchen et al., with nonspecific INR, is an understandable but less reliable approach to EQA of the POCT PT monitors. Thus the smaller number of unsatisfactory performances detected by UK NEQAS compared with ECAA/EAA studies and the lower detection rate of unsatisfactory CoaguChek test strips is easily explained.
Acknowledgments
Grant/funding support: The work was supported by grants from the EC Commission (SMT4-CT98-2269 and QLG4-CT-2001-02175) and from the Manchester Thrombosis Research Foundation.
Financial interests: None declared.
References
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