A 65-year-old male with documented atrial flutter who was taking warfarin chronically returned to the anticoagulation clinic for follow-up, after having been on 10 mg daily for approximately 2 weeks. He had a previous sub-therapeutic international normalized ratio of 1.7 on a dose of 65 mg/week. The international normalized ratio at this visit was now 4.77 via venipuncture, after just an 8% increase in weekly dosing. He self-reported adherence to the new warfarin dosing but had begun eating grapefruit since last visit. The patient had no active bleeding and was told to decrease his dose to 8 mg daily. He also stopped eating the grapefruit. One week later, he returned to the clinic and the international normalized ratio was 2.1. He is currently back on warfarin 65 mg/week, and his international normalized ratio has been within therapeutic range for the past 4 months. Clinicians should have a heightened awareness of the potential for elevated international normalized ratio when grapefruit juice is consumed in a patient who is taking warfarin.
Keywords: Warfarin; elevation; enzymes; grapefruit; international normalized ratio.