European concerted action on anticoagulation. Use of plasma samples to derive international sensitivity index for whole-blood prothrombin time monitors.

BACKGROUND To simplify International Sensitivity Index (ISI) calibration, the possibility of substituting fresh plasma for fresh whole-blood samples with point-of-care testing (POCT) whole-blood monitors was investigated in a three-center study of three different POCT systems. METHODS A modified full WHO calibration procedure based on 20 healthy controls and 60 coumarin-treated patients was performed on three monitoring systems with whole-blood and plasma samples against plasma tested using the European Concerted Action on Anticoagulation (ECAA) rabbit reference plain thromboplastin and the manual prothrombin time (PT) method. RESULTS With one of the three systems, the mean ISI was 1.51 for whole blood and 1.49 for plasma; with the second system, the mean ISI was 1.08 for both whole blood and plasma. With the third system, however, the difference between the mean ISI for whole blood and that for plasma was greater (1.15 and 1.01, respectively). Overall, the precision of the calibrations was less than with traditional manual plasma PT testing. CONCLUSIONS Provided that an appropriate calcium chloride concentration is used, the plasma PT results can be used for accurate ISI calibration of two of these three whole-blood POCT systems. Precision criteria need to be modified for POCT monitors.