Early mechanical mitral valve thrombosis in a patient with warfarin resistance.

Warfarin, an oral anticoagulant, is the therapy of choice to maintain anticoagulation. An individual requiring five- to 20-fold higher dosage than average for anticoagulation may be considered as having resistance to warfarin. In order to evaluate a subtherapeutic response to high-dose warfarin, the clinician must consider many possible causes of resistance, such as non-compliance, drug interactions, or pharmacokinetic changes. When these factors have been eliminated, an hereditary warfarin resistance might be considered responsible. The case is reported of a 49-year-old woman who received warfarin after mitral valve replacement and experienced mechanical mitral valve thrombosis due to inadequate anticoagulation, possibly caused by warfarin resistance.