Investigation of patients with abnormal response to warfarin.

In 55 patients in whom warfarin control had been satisfactory for at least 4 months, warfarin dose, plasma warfarin concentration and plasma clearance were measured. The mean dose to maintain the BCR (INR) between 2.3 and 3.3 was 5.1 +/- 2 mg day-1 (range 1.5-10 mg). Plasma warfarin concentrations and warfarin clearance were log-normally distributed with 95% confidence limits between 0.8 and 2.4 mg l-1 and 2.5 and 8.71 day-1 respectively. These confidence limits were used to construct algorithms which correctly predicted the cause of abnormal warfarin sensitivity in two patients and resistance in two further patients. These algorithms should help to identify the cause of abnormal warfarin responsiveness in the clinical setting.