INR 5.2 After Starting Trimethoprim-Sulfamethoxazole
Mr. D is a 68-year-old man with atrial fibrillation taking warfarin 5 mg daily. His INR has been stable between 2.4 and 2.8 for six months. He also takes amiodarone 200 mg daily, metoprolol 50 mg BID, and atorvastatin 40 mg nightly. He presents to the hospital emergency department with two days of dysuria and urinary frequency. Urine culture grows Escherichia coli susceptible to trimethoprim-sulfamethoxazole. The emergency physician prescribes TMP-SMX double-strength (160/800 mg) one tablet BID for seven days. Four days later, his INR is 5.2. He has no gross bleeding, but reports mild bruising on his forearms. His hemoglobin is 132 g/L and platelets are normal. He lives in a rural area with INR monitoring available only twice weekly. He is on day 4 of the antibiotic and asks whether he should stop taking warfarin.
1. Explain the likely mechanisms for the elevated INR and identify contributing factors.
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- TMP-SMX inhibits CYP2C9, the enzyme responsible for metabolism of S-warfarin, and can also displace warfarin from protein binding and reduce gut flora that produce vitamin K.
- Amiodarone inhibits CYP2C9 and CYP3A4 and has a long half-life, so it already increases warfarin exposure.
- Acute infection and reduced oral intake may further lower vitamin K intake.
- The INR rise is a predictable interaction, not an idiosyncratic reaction. The mild bruising is a warning sign even without gross bleeding.
2. Outline your immediate management, including whether to hold warfarin, use vitamin K, or change antibiotics.
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- Withhold the next 1–2 warfarin doses and recheck INR in 24–48 hours.
- Do not give vitamin K routinely for INR 5.2 without bleeding; reserve oral vitamin K 1–2.5 mg for bleeding or very high bleeding risk, per local protocol.
- Assess for bleeding: bruising, hematuria, melena, headache, neurological symptoms.
- Avoid aspirin, NSAIDs, and other antiplatelets unless indicated.
- Consider changing the antibiotic based on culture: nitrofurantoin if renal function allows, fosfomycin single dose, or another susceptible agent. If TMP-SMX must continue, reduce the warfarin dose and monitor closely.
- Do not simply stop warfarin permanently; the patient needs anticoagulation for atrial fibrillation.
3. What monitoring and discharge plan do you recommend for this rural patient?
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- Arrange INR testing every 2–3 days until stable, then weekly, given limited rural access.
- Adjust the warfarin dose by about 10–20% or use a temporary hold, following a validated nomogram.
- Document the interaction and the plan in the chart and communicate with the family physician.
- Educate the patient: report bleeding, avoid new over-the-counter NSAIDs, maintain consistent vitamin K intake, and inform all prescribers about warfarin.
- Confirm follow-up arrangements before discharge.