Warfarin, a New Antibiotic, and an INR of 7.2
A 70-year-old woman with atrial fibrillation has been taking warfarin 5 mg daily; her INR was 2.6 two weeks ago. Four days ago a walk-in clinic prescribed trimethoprim-sulfamethoxazole for a urinary tract infection. Today she reports gum bleeding when brushing and one black stool. Vital signs: blood pressure 112/70, heart rate 88, respirations 18. Laboratory results: INR 7.2, hemoglobin 10.8 g/dL, platelets 180,000/µL, creatinine 1.1 mg/dL. She has no active visible bleeding now and denies headache, hematuria, or abdominal pain. The covering clinician orders warfarin 5 mg by mouth now and vitamin K 10 mg IV now. The patient asks, “Should I stop the antibiotic?”
1. Which order(s) do you question or hold, and why?
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I would hold the warfarin. The INR is 7.2 with gum bleeding and a black stool, so the risk of major bleeding is real. I would question the order for vitamin K 10 mg IV: IV vitamin K is for serious or life-threatening bleeding and can cause anaphylactoid reactions and prolonged warfarin resistance. With no active major bleeding, a small oral dose (for example 1–2.5 mg) per protocol is often preferred, and the clinician must specify. I would notify the covering clinician before giving either medication, state the INR and melena, and request a plan. I would not give the antibiotic advice without checking; trimethoprim-sulfamethoxazole inhibits warfarin metabolism and is the likely cause.
2. What monitoring and patient teaching take priority?
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Monitor for bleeding: vital signs, mental status, urine, stool, gums, skin, and abdominal pain. Repeat INR and CBC, check renal function, and type and screen if bleeding worsens. Teach the patient to avoid NSAIDs and aspirin unless prescribed, to report black stools or blood in urine, and to keep vitamin K–rich foods consistent rather than eliminating them. She should not stop the antibiotic on her own; the prescriber needs to choose an alternative or adjust warfarin with close INR follow-up. I would arrange a repeat INR in 24–48 hours and ensure she has a phone number for bleeding symptoms. Fall precautions and a soft toothbrush are appropriate.
3. How do you communicate with the covering clinician and document?
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I would use SBAR: “70-year-old on warfarin, started TMP-SMX 4 days ago; INR 7.2, gum bleeding, one black stool, Hgb 10.8, hemodynamically stable. You ordered warfarin 5 mg and vitamin K 10 mg IV. I am holding warfarin and questioning the IV vitamin K dose/route because there is no major bleeding. Please advise on oral vitamin K and repeat INR.” I would document the notification, the orders questioned, the patient’s response, and the plan. If the clinician insists on 10 mg IV, I would clarify the indication and escalate to the pharmacist or attending. The documentation should show that the interaction was identified and acted on.