Familial warfarin resistance.

Warfarin, a coumarin anticoagulant, acts by interfering with the hepatic synthesis of the vitamin-K-dependent clotting factors. It is used clinically in the treatment or prophylaxis of venous thrombosis and embolisms. Resistance to the effects of the coumarin and indanedione anticoagulants has been reported in rats and man, but its incidence has been defined as extremely rare. Resistance has been described as relative, acquired, or hereditary. The first well-documented case was also the first report to identify a genetic basis for this resistance. Since that time, there has been only one other study that strengthened the evidence for a hereditary transmission, and only a few other reports have suggested hereditary influence as a reason for coumarin resistance. In this report, a patient who presented with a familial-type warfarin resistance is described. A discussion of previous reports and possible mechanisms for nonfamilial warfarin resistance is also included.