Abstract
Warfarin therapy has traditionally been monitored using the prothrombin time (PT). A significant problem with this assay is the variable sensitivity of commercially available thromboplastin reagents to reduction of vitamin K-dependent coagulation factors. The International Normalized Ratio (INR) was developed as a means of standardizing PT values between laboratories that use different thromboplastins and types of instrumentation. Parallel testing of samples from 63 patients stabilized on oral anticoagulation with five different thromboplastins was undertaken. Forty-eight percent of all samples had INR values that were not identical but showed good correlation. Fifty-two percent of the samples had clinically significant discrepancies of their INR values. In this group, patients in the therapeutic range with one thromboplastin appeared over- or underanticoagulated based on the INR using a different thromboplastin. In order to determine whether use of a low-ISI thromboplastin reagent could provide more reproducible INR results, concurrent testing of specimens from 36 patients on stable oral anticoagulation was undertaken between our hospital laboratory and another, nonaffiliated institution. Both hospitals used similar instrumentation and thromboplastin reagents. Under these conditions, the INR values generated between laboratories correlated highly. The inability to generate consistent INR values in parallel testing using different thromboplastins with identical samples and instrumentation raises concern about the reliability of the INR, and suggests that further analysis is necessary to identify the source of these discrepancies. In the interim, our data show use of a recombinant thromboplastin with a low-ISI value substantially improves interlaboratory variation in INR value. Key Words: International Normalized Ratio—Prothrombin time—Thromboplastin—Anticoagulation monitoring.
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