Warfarin resistance in a patient with mechanical valve prosthesis.

he term warfarin resistance is usually reserved for patients who require warfarin in excess of 20 mg/day to maintain the International Normalized Ratio (INR) within the therapeutic range. It is usually encountered at the start of therapy, however, it may also occur in patients who have been already on warfarin therapy. Both diet and the co-administration of certain drugs can have a marked effect on the anticoagulant efficacy of warfarin. A 45-year-old lady who had a mechanical mitral valve prosthesis (Tilting Disc Prosthesis) 12 years ago presented with chest pain and shortness of breath on moderate exertion. She was maintained on warfarin until 3 years ago when she developed left hemiparesis. Since then there have been difficulty in getting her adequately anticoagulated and she was labeled as having "warfarin resistance". Despite giving her up to 35 mg of warfarin a day, her INR never reached a therapeutic level. She has not been on any medications that could have interfered with the action of warfarin. Subsequently, she was shifted to enoxaparin 80 mg subcutaneous injection twice a day and was even offered to have her