INR 6.4 After a Common Antibiotic: Does the Warfarin Go In?
Mr R, 74 and 68 kg, has atrial fibrillation and takes warfarin 5 mg daily (target INR 2.0–3.0). Five days ago he started trimethoprim–sulfamethoxazole 960 mg twice daily for a urinary tract infection. Other medicines: amiodarone 200 mg daily, furosemide 40 mg daily, omeprazole 20 mg daily. His INR today is 6.4; it was 2.6 at the anticoagulation clinic two weeks ago. eGFR 55 mL/min/1.73 m². He looks well, denies bleeding, has no bruising or gum bleeding, urine is clear, no headache, and Hb is 128 g/L (unchanged). The ward doctor has written: "Warfarin 5 mg PO now. Repeat INR in one week." The warfarin is due in ten minutes and he is asking why he is having another blood test.
1. Explain why the INR has risen. What do you do about the dose that is due in ten minutes, and why?
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Mechanism - three additive effects:
- Trimethoprim–sulfamethoxazole inhibits CYP2C9 (the main enzyme metabolising S-warfarin) and also displaces warfarin from albumin. It is one of the best-recognised precipitants of dangerous over-anticoagulation, and the effect appears within days - exactly the timeline here.
- Amiodarone is a long-half-life CYP2C9/CYP3A4 inhibitor and typically requires a warfarin dose reduction of 30–50% when started; it is a chronic contributor.
- Age, low body weight, reduced renal function (eGFR 55) and the acute infection itself (reduced vitamin K intake, acute phase response) all push the INR up.
Do not give the 5 mg dose. An INR of 6.4 is far above a therapeutic range of 2.0–3.0; giving a further 5 mg continues the error while the drug effect persists for days. Withhold the dose, keep it safe and available (do not discard), and contact the prescriber now for a revised plan. The written order is not a mandate to give a drug that has become unsafe - the nurse is the last check.
Explain to the patient simply: "Your blood is thinner than we want because of the antibiotic you are taking. We are stopping the warfarin for a day or two and will recheck." Do not describe it as an error if it is a predictable interaction - but do ensure he knows to report antibiotic courses in future.
2. What is the correct management of an INR of 6.4 with no bleeding, and what exactly would you request from the prescriber?
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Standard approach for INR above therapeutic range but with no bleeding:
- Withhold warfarin (already done) and identify/correct the precipitant - in this case, review whether the TMP-SMX can be changed to a non-interacting antibiotic (e.g. nitrofurantoin if renal function permits, or an alternative guided by culture).
- Consider low-dose oral vitamin K 1–2.5 mg for an INR above about 6–10 without bleeding, per local protocol. Oral route is preferred: it is safer and does not cause the anaphylactoid risk of IV phytomenadione, and it avoids over-correction and later warfarin resistance.
- Recheck INR in 24–48 hours, not one week. An INR of 6.4 can rise further; a one-week interval leaves him unprotected from bleeding.
- Do not give prothrombin complex concentrate or FFP - these are for major bleeding or an INR with bleeding, and would expose him to unnecessary thrombotic/volume risk.
- Safety-net advice: avoid NSAIDs and aspirin, avoid alcohol binges, use a soft toothbrush, and return immediately for黑 stool, blood in urine, unusual bruising, or headache (intracranial bleed).
What to request, in SBAR form: "Mr R, 74, on warfarin, INR 6.4 today from 2.6, no bleeding, Hb stable, day 5 of co-trimoxazole. I have withheld tonight's warfarin. Please review: can we change the antibiotic, do you want oral vitamin K 1–2 mg, and can we recheck INR in 24–48 hours? Also, should amiodarone be reviewed as a chronic interaction?"
Document the withheld dose, the reason, who was informed and when, and the monitoring plan.
3. He asks whether he should just stop the warfarin himself and whether he can "eat more greens to fix it". How do you respond, and what else on his medication list needs attention?
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Responding to the patient:
- Never stop warfarin on his own - the reason he takes it is stroke prevention in atrial fibrillation; an abrupt stop in the context of a high INR is not the same as safe discontinuation, and rebound thrombosis risk is real. The decision and the duration of the hold belong to the prescriber.
- Do not use diet as an acute treatment. A sudden increase in vitamin K-rich foods (kale, spinach, broccoli) will lower the INR unpredictably over days and destabilise control; the aim is consistency, not a spike. This is a common misconception worth correcting explicitly.
- A useful teaching point: "It's not the greens themselves, it's big changes in amount. Keep eating what you usually eat, and tell me if that changes."
Other medication issues to flag:
- Co-trimoxazole in reduced renal function (eGFR 55) - check dose appropriateness, review the culture, and consider a switch to a non-interacting agent. Co-trimoxazole also carries hyperkalaemia risk with any ACE inhibitor/ARB and can cause hypoglycaemia in older adults.
- Omeprazole inhibits CYP2C19 and can modestly increase INR; it also reduces B12 and magnesium with long-term use. Not the main culprit but part of the picture.
- Furosemide - check U&E and potassium; dehydration and acute illness raise INR.
- Falls and bleeding risk: screen for falls, check whether he has a home INR monitoring option, and record a baseline neuro observation plan if he develops a headache.
Discharge teaching (teach-back): carry an anticoagulant card, tell every prescriber and dentist, avoid self-medicating with new drugs including herbal remedies, know the bleeding warning signs, and keep the INR appointments. Confirm understanding by asking him to explain back what he will do if he notices blood in his urine.