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 physicians to question the reliability of the INR.1 This is ironic, because it was the introduction of the INR system that brought to light some of the long-standing problems with the technique of PT monitoring. However, these problems are not insurmountable if a compromise can be reached between the expectations of laboratory physicians and of clinicians. Thus, the laboratory physician seeks a perfect assay system, which in the case of the INR is unattainable at present, because of differences in PT reagents and methods. In contrast, the clinician is satisfied with a system of monitoring that provides safe and effective warfarin 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 solutions are offered.