The use and limitations of the INR system

This study was undertaken to answer some practical questions physicians and medical directors of clinical laboratories face when they contemplate replacing their high international sensitivity index (ISI) thromboplastins 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 produce a prolonged PT on normal patients, the answer is probably no. To the question of the extent of normalization of disparate PTs, determined by high and low ISI thromboplastins, of patients on oral anticoagulants 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 markedly, but not completely, normalize the PTR values. In other words, there would be a lessening of disparity of the PTR after the conversion. For patients whose PTs were > 20 sec, conversion of PTR to INR could even widen the disparity seen with the PTR. Finally, when PTs were assayed on different coagulation devices with the same reagent, highly congenial results were obtained.