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 (International Sensitivity Index [ISI] < or = 1.2) provide the most consistent performance and minimize interlaboratory variability. We compared the INR values obtained from 69 specimens drawn from patients receiving long-term oral anticoagulant therapy, using four thromboplastin preparations (manufacturer-assigned ISI range of 0.96-1.10) and two automated photo-optical analyzers. Multivariate analysis of the INR response matrix (552 INR values) indicated that the eight reagent-coagulometer combinations did not produce equivalent INR values. Similar analysis indicated that INR values were not normalized when uncorrected 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 INR values were assigned to one of four therapeutic categories used in oral anticoagulant therapy (< 2.0; 2.0-3.0; 3.0-4.5; or > 4.5). These differences in INR values obtained with two photo-optical coagulometers and four highly sensitive thromboplastin reagents suggest that the existing INR system has not achieved the goal of standardized prothrombin time values and does not support the recommendation to use only highly sensitive reagents for the regulation of oral anticoagulant therapy.