Hereditary warfarin resistance. Investigation of a rare phenomenon.

A 57-year-old black woman required a daily dosage of 50 mg of warfarin sodium to maintain her prothrombin time in a therapeutic range. The central volume of distribution and clearance of warfarin were normal for this patient. These findings, combined with the patient's requirement for plasma warfarin levels four times greater than those usually required to achieve adequate anticoagulation, indicated that the relative resistance was due to altered pharmacodynamics of warfarin. The only child of the propositus, a daughter, showed a similar relative resistance, confirming that this family is the third to be reported with hereditary resistance to warfarin.