SYSTEM ISI CALIBRATION - A UNIVERSALLY APPLICABLE SCHEME IS POSSIBLE ONLY WHEN COUMARIN PLASMA CALIBRANTS ARE USED

Coagulometer-induced errors in International Normalized Ratios (INR) m ay 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 deter mined the INR of eight frozen coumarin plasmas. There was considerable vari ation in INR reported at the different centres, with coefficients of v ariation (CV) ranging from 11.30% to 17.29%. In addition, their consen sus mean INR values were consistently higher than those obtained manua lly. The overall mean percentage deviation was 20.06%, indicating that correction was required to reduce the over-estimation of INR that occ urred even in those Systems incorporating a thromboplastin reagent wit h an instrument-specific ISI. Local correction using coumarin plasma c alibrants reduced this value to less than or equal to 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 empl oyed, mean percentage deviations were 7.87% with BCT 441, 12.47% with OBT 79 and 77.37% with RBT 90 as reference respectively. This differen ce may be due, in part, to the difficulty in assigning an INR to adsor bed plasma calibrants, which contain no protein induced by vitamin K a ntagonists (PIVKA). This study demonstrates that coumarin plasma calib rants are superior in System ISI calibration.