ABSTRACT
A 56-year-old man is referred for anti-coagulation with warfarin as he has atrial fibrillation. His only other medication is frusemide. However, it proves very difficult to increase the international normalised ratio (INR) above 1.5 and 4 months after commencing warfarin his INR is 1.4 despite being prescribed 25 mg of warfarin daily for the last fortnight. The patient insists that he is compliant with his medication and religiously takes the correct dose at the same time each day.
