Supratherapeutic INR After Trimethoprim-Sulfamethoxazole
A 76-year-old man with atrial fibrillation takes warfarin 5 mg daily, and his INR has been stable between 2.2 and 2.8 for 14 months. He presents to urgent care with dysuria and frequency. Urinalysis shows nitrite-positive, leukocyte esterase-positive urine, and the clinician prescribes trimethoprim-sulfamethoxazole DS twice daily for 7 days. He also takes amlodipine, metoprolol, and atorvastatin; his eGFR is 52 mL/min/1.73 m². Four days later he returns with gum bleeding after brushing his teeth and several small bruises. He is hemodynamically stable, has no melena or hematuria, and his hemoglobin is 12.9 g/dL (baseline 13.4). His INR is 6.7. He asks whether he can just stop the antibiotic and continue his usual warfarin dose.
1. Explain the drug interaction and its mechanism.
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Trimethoprim-sulfamethoxazole is a classic potentiator of warfarin. Sulfamethoxazole inhibits CYP2C9, the main enzyme metabolizing S-warfarin, and both components can displace warfarin from albumin. Trimethoprim is renally cleared, and an eGFR of 52 mL/min/1.73 m² causes accumulation. The INR typically rises within 3–5 days, which matches his presentation. The differential includes other new drugs, dietary vitamin K change, alcohol use, and adherence, but the temporal relationship is compelling. This is not an idiosyncratic reaction; it is predictable. For future infections, choose an agent with minimal CYP2C9 interaction and appropriate renal dosing. Nitrofurantoin 100 mg twice daily for 5 days is reasonable for uncomplicated cystitis at this eGFR, or fosfomycin 3 g as a single dose. If trimethoprim-sulfamethoxazole is unavoidable, preemptively reduce the warfarin dose and check the INR at 3 and 5–7 days.
2. What is the appropriate management of the INR of 6.7 with minor mucosal bleeding?
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He has minor mucosal bleeding, not major bleeding, so do not give fresh frozen plasma or prothrombin complex concentrate. Hold warfarin. For an INR of 6.7 with minor bleeding, give oral vitamin K 1–2.5 mg, not 10 mg, and recheck the INR in 24 hours; high-dose vitamin K can cause warfarin resistance and prolonged instability. If there were no bleeding at all, guidelines would support holding warfarin without routine vitamin K for an INR of 4.5–10. Stop the trimethoprim-sulfamethoxazole or switch to a non-interacting antibiotic. When the INR falls below 3, resume warfarin at a reduced dose-roughly 3–4 mg rather than 5 mg-and recheck the INR in 3–7 days. Do not simply continue the same warfarin dose, because the interaction persists for days after the antibiotic is stopped. Check hemoglobin and assess for occult bleeding, and counsel him on bleeding precautions.
3. After this episode, how should long-term anticoagulation be reassessed, and when would a direct oral anticoagulant be preferred?
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First address the acute problem, then reassess the overall anticoagulation plan. Calculate his CHA₂DS₂-VASc score to confirm that anticoagulation is indicated and his HAS-BLED score to identify modifiable bleeding risks. Direct oral anticoagulants are preferred over warfarin for most patients with nonvalvular atrial fibrillation because of fewer drug and food interactions and a lower risk of intracranial hemorrhage. They are not appropriate for mechanical heart valves or moderate-to-severe mitral stenosis. For him, apixaban 5 mg twice daily would be a common choice; the reduced 2.5 mg dose requires two of the following: age 80 or older, weight 80 kg or less, or creatinine 1.5 mg/dL or higher. Check his renal function, weight, and age before dosing. If he remains on warfarin, arrange more frequent INR checks after any antibiotic and consider an anticoagulation clinic. Discuss cost, adherence, and preference.