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.

The Journal of Heart Valve Disease 2007;16:200-202