Kj. Stevenson et al., SYSTEM ISI CALIBRATION - A UNIVERSALLY APPLICABLE SCHEME IS POSSIBLE ONLY WHEN COUMARIN PLASMA CALIBRANTS ARE USED, British Journal of Haematology, 96(3), 1997, pp. 435-441
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.