THE INTERNATIONAL NORMALIZED RATIO - A GUIDE TO UNDERSTANDING AND CORRECTING ITS PROBLEMS

With the increasing use of the international normalized ratio (INR) to monitor warfarin therapy, a number of problems with prothrombin time (PT) testing have been identified that have led some laboratory physic ians to question the reliability of the INR.(1) This is ironic, becaus e it was the introduction of the INR system that brought to light some of the long-standing problems with the technique of PT monitoring. Ho wever, these problems are not insurmountable if a compromise can be re ached between the expectations of laboratory physicians and of clinici ans. Thus, the laboratory physician seeks a perfect assay system, whic h in the case of the INR is unattainable at present, because of differ ences in PT reagents and methods. In contrast, the clinician is satisf ied with a system of monitoring that provides safe and effective warfa rin dosing. This goal can be achieved provided that certain details of PT testing are observed. In this communication, which is directed to practicing clinicians, the potential problems with the INR system are discussed, their clinical relevance is critically reviewed, and soluti ons are offered.