Warfarin, a New Antibiotic, and a Supratherapeutic INR
Mrs. D is 82 with atrial fibrillation, takes warfarin 4 mg daily, target INR 2.0–3.0. Two days ago she started trimethoprim-sulfamethoxazole for a urinary tract infection. Today she reports gum bleeding when brushing and new bruising on both forearms. Her INR is 5.2, hemoglobin 108 g/L, and she has no melena, hematuria, or severe headache. She takes ibuprofen 400 mg occasionally for knee pain and ginkgo daily, both purchased over the counter. She lives alone and drives. The prescriber is at lunch and the pharmacy is closed for another hour. She asks if she should skip her warfarin dose today. Her last fall was two years ago. She has mild knee osteoarthritis and no history of bleeding. She has no caregiver at home.
1. What is the nurse's immediate priority?
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Hold the next warfarin dose, notify the prescriber, and assess for bleeding: gums, bruising, melena, hematuria, headache, and neurological change. Check CBC, INR, and renal function as ordered. Do not give IM injections or NSAIDs. Apply pressure to the gums and use fall precautions. Vitamin K may be ordered depending on INR and bleeding; the nurse cannot give it independently. Recheck INR after intervention. Her INR of 5.2 with minor bleeding is serious but not yet an emergency if she is stable; escalate promptly.
2. Explain the likely cause of the elevated INR.
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Trimethoprim-sulfamethoxazole is a potent warfarin interaction: it inhibits CYP2C9 metabolism and can displace warfarin from albumin, raising INR within days. Ibuprofen adds an antiplatelet effect and gastric irritation; ginkgo may also increase bleeding. Age and acute infection contribute. The nurse should obtain a full medication history, including over-the-counter drugs and herbals, and flag the interaction. This is not just that the INR is high; the cause changes the teaching and monitoring plan.
3. What discharge teaching is essential?
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Teach her to avoid ibuprofen and ginkgo, and to ask before taking any over-the-counter product. She must tell every prescriber and pharmacist that she takes warfarin. Use a soft toothbrush and electric razor, and report black stools, blood in urine, unusual headache, or bleeding that will not stop. Keep vitamin K intake consistent, not eliminated. Do not double a missed dose. Arrange INR monitoring and a medication review; because she lives alone and drives, assess for dizziness or bleeding risk.