System ISI calibration: a universally applicable scheme is possible only when coumarin plasma calibrants are used

Coagulometer-induced errors in International Normalized Ratios (INR) may be corrected by assigning an International Sensitivity Index (ISI) specific for the local combination of coagulometer and thromboplastin reagent (System ISI). In the present study, 57 laboratories determined the INR of eight frozen coumarin plasmas. There was considerable variation in INR reported at the different centres, with coefficients of variation (CV) ranging from 11.30% to 17.29%. In addition, their consensus mean INR values were consistently higher than those obtained manually. The overall mean percentage deviation was 20.06%, indicating that correction was required to reduce the over-estimation of INR that occurred even in those Systems incorporating a thromboplastin reagent with an instrument-specific ISI. Local correction using coumarin plasma calibrants reduced this value to < or = 7% with current reference thromboplastins BCT 441, OBT 79 and RBT 90 and also lowered the CV to < 5%. In contrast, when adsorbed plasma calibrants were employed, mean percentage deviations were 7.87% with BCT 441, 12.47% with OBT 79 and 77.37% with RBT 90 as reference respectively. This difference may be due, in part, to the difficulty in assigning an INR to adsorbed plasma calibrants, which contain no protein induced by vitamin K antagonists (PIVKA). This study demonstrates that coumarin plasma calibrants are superior in System ISI calibration.