Failure to Generate Comparable International Normalized Ratio Values Using Five Different Thromboplastin Reagents in Parallel Studies of Patients Receiving Warfarin

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 repr...