EFFECT OF PLASMA POOLING ON THE INTERNATIONAL-SENSITIVITY-INDEX OF PROTHROMBIN TIME SYSTEMS

Guidelines set by the World Health Organization (WHO) state that in or der to calibrate a prothrombin time system for International Sensitivi ty Index (ISI), freshly prepared specimens from 20 normal individuals and 60 patients receiving coumarin are required. These numbers are req uired to obtain a precise value of the calibration line slope when the re is considerable scatter of individual data about the regression lin e. The scatter can be reduced by pooling individual plasma samples. In the present study, four pooled plasmas were prepared, one from 20 nor mal individuals and three from three groups of 30 patients receiving t reatment with long-term oral anticoagulants. Prothrombin times were de termined with four thromboplastins, HepatoQuick (rabbit thromboplastin combined with adsorbed plasma), Recombiplastin (recombinant human thr omboplastin), Thromborel-S (human placenta), and Thromboplastin-C Plus (rabbit brain). Calibration line slopes were calculated for the six p ossible combinations of thromboplastins using the set of all individua l plasma samples and the set of four pooled plasmas. In most compariso ns, the WHO calibration model was appropriate, i.e. the line calculate d for the patients' samples passed through the mean of the normals. Th e calibration line slopes obtained with the set of four pooled plasmas did not differ by more than 5% from the corresponding slopes calculat ed with the original individual plasmas. For some combinations of thro mboplastins non-linear relations were observed both with the individua l plasmas and with the pooled plasmas. We conclude that pooling indivi dual plasmas does not significantly change the calibration relation be tween prothrombin times determined with the original individual plasma s. Freshly pooled plasmas can be used to determine the ISI of prothrom bin time systems with an acceptable degree of precision. (C) 1998 Lipp incott Williams & Wilkins.