HIGHLY SENSITIVE THROMBOPLASTINS DO NOT IMPROVE INR PRECISION

For determination of the international normalized ratio (INR), it has been suggested that ''highly sensitive'' thromboplastin reagents (Inte rnational Sensitivity Index [ISI] less than or equal to 1.2) provide t he most consistent performance and minimize interlaboratory variabilit y. We compared the INR values obtained from 69 specimens drawn from pa tients receiving long-term oral anticoagulant therapy, using four thro mboplastin preparations (manufacturer-assigned ISI range of 0.96-1.10) and two automated photo-optical analyzers. Multivariate analysis of t he INR response matrix (552 INR values) indicated that the eight reage nt-coagulometer combinations did not produce equivalent INR values. Si milar analysis indicated that INR values were not normalized when unco rrected prothrombin ratios or INR values, calculated after assignment of ''local ISI values'' to each thromboplastin reagent, were compared. The INR differences also seemed to be clinically significant because 17% to 29% of paired thromboplastin values were discordant when all IN R values were assigned to one of four therapeutic categories used in o ral anticoagulant therapy (< 2.0; 2.0-3.0; 3.0-4.5; or > 4.5). These d ifferences in INR values obtained with two photo-optical coagulometers and four highly sensitive thromboplastin reagents suggest that the ex isting INR system has not achieved the goal of standardized prothrombi n time values and does not support the recommendation to use only high ly sensitive reagents for the regulation of oral anticoagulant therapy .