Exaggerated warfarin sensitivity: a case report

A 64-y-old male was hospitalized because of significant bleeding and a prolonged prothrombin time (PT) (greater than 200 sec) following an inadvertent doubling of his anticoagulant dose. He was previously well controlled on 2.5 mg warfarin every fourth day. Treatment included vitamin K, fresh frozen plasma, and packed red blood cells. Warfarin plasma concentrations, clotting factor analysis (CFA) and PTs were analyzed to evaluate this patient. Following hospitalization, the patient was followed by the Anticoagulation Clinic and his dose was stabilized at 0.5 mg daily. He experienced no further bleeding episodes and his PT ratio was maintained at 1.5 times control. The response to a dose of warfarin varies greatly from patient to patient. Previous reports of abnormal responses to warfarin can be categorized as either warfarin "resistant" or "sensitive." The preponderance of reports are compliance or hereditary warfarin resistance. This patient represents a case of exaggerated warfarin sensitivity that cannot be easily explained. There was no evidence of a drug interaction, warfarin concentrations were not excessive, and CFA was reflective of a moderate warfarin effect. Extreme tissue sensitivity to warfarin is a plausible explanation for this abnormal response.