Complications of warfarin therapy monitored by the international normalized ratio versus the prothrombin time ratio

The objective of our study was to determine the rates of bleeding complications and thromboembolic events in patients receiving oral anticoagulant therapy monitored with the prothrombin time (PT) ratio versus therapy monitored with the International Normalized Ratio (INR) using a retrospective time‐series study design. Over 650 patients enrolled in a large anticoagulation clinic were studied during two time periods corresponding to the use of the PT ratio versus the INR to guide anticoagulant therapy, with over 400 patient‐years of follow‐up for each time period. The rate of bleeding complications using the PT ratio to guide therapy was 6.7% (1.2% major, 5.5% minor) per patient‐year, compared with 2.9% (0% major, 2.9% minor) using the INR (p = 0.02). The rate of thromboembolic complications was 1.0% using the PT ratio, compared with 0.2% using the INR (p = NS). Therapy monitored with the INR required 19.8 visits per year, compared with 20.7 visits per year using the PT ratio. We conclude that the INR should be used to monitor oral anticoagulant therapy in an effort to reduce bleeding complications while maintaining an acceptable rate of thromboembolic events.