Bob, a 65-year-old man with atrial fibrillation, has been using warfarin for the past 12 months after he presented

Bob, a 65-year-old man with atrial fibrillation, has been using warfarin for the past 12 months after he presented to the local emergency department with signs of a TIA. A head CT scan and trans-esophageal echocardiogram done at the time were normal. He has been well since. Bob has came it the clinical today as the INR taken this morning was 4.6. Up until now, his INR results (which have been measured every 2 weeks) have been stable and in the range of 2.0√3.0. He has not started any new prescribed medications recently. Bob also has hypertension and osteoarthritis (for which he had a left total hip replacement 6 months ago). Current medications: Atenolol 50 mg once daily, Lisinopril 10 mg once daily and warfarin 6 mg at night. On examination: BP 140/80 mmHg; pulse rate 65, irregular. The remainder of the physical examination is normal with no evidence of bruising, epistaxis, or gastrointestinal bleeding.