Poor Agreement among Prothrombin Time International Normalized Ratio Methods: Comparison of Seven Commercial Reagents

most popular test for monitoring oral anticoagulation therapy. The International Normalized Ratio (INR) was introduced to overcome the problem of marked varia-tion in PT results among laboratories and the various recommendations for patient care. According to this principle, all reagents should be calibrated to give identical results and the same patient care globally. This is necessary for monitoring of single patients, and for application of the results of anticoagulation trials and guidelines to clinical practice. Methods: We took blood samples from 150 patients for whom oral anticoagulation had been prescribed. Plas-mas were separated and PTs determined by use of seven commercial reagents and four calibrators. The differ-ences in results were assessed by plotting, for eac