THE USE AND LIMITATIONS OF THE INR SYSTEM

This study was undertaken to answer some practical questions physician s and medical directors of clinical laboratories face when they contem plate replacing their high international sensitivity index (ISI) throm boplastins with low ISI ones and using international normalized ratio (INR) in place of prothrombin time ratio (PTR) for monitoring warfarin therapy. To the question of whether low-ISI thromboplastins would pro duce a prolonged PT on normal patients, the answer is probably no. To the question of the extent of normalization of disparate PTs, determin ed by high and low ISI thromboplastins, of patients on oral anticoagul ants upon the conversion of PTR to INR, the answer is a mixed one. For those whose PTs were 14-20 sec, conversion of PTR to INR would marked ly, but not completely, normalize the PTR values. In other words, ther e would be a lessening of disparity of the PTR after the conversion. F or patients whose PTs were >20 sec, conversion of PTR to INR could eve n widen the disparity seen with the PTR. Finally, when PTs were assaye d on different coagulation devices with the same reagent, highly conge nial results were obtained. (C) 1994 Wiley-Liss, Inc.