Warfarin, the major oral anticoagulant currently available in Taiwan, is used for the prevention and treatment of a variety of thromboembolic disorders. Under dosing of warfarin may cause treatment failure and over dosing may cause bleeding because of its narrow therapeutic index. Therefore, it is important to monitor prothrombin time (PT) and international normalize ratio (INR) periodically in order to evaluate the appropriateness of warfarin dosing.