INR 7.8 After Co-trimoxazole in a Mechanical Mitral Valve

Warfarin is a racemic mixture; the more potent S-enantiomer is cleared by CYP2C9. Sulfamethoxazole in co-trimoxazole is one of the strongest CYP2C9 inhibitors in common outpatient use, and trimethoprim adds further inhibition; both are also protein-bound and can displace warfarin. The result is reduced clearance of S-warfarin and a rising INR within 3–7 days - the delay reflects the half-lives of the vitamin K-dependent clotting factors, with factor VII falling first and prothrombin later. That timing fits exactly: INR 3.1 two weeks ago, a five-day antibiotic course starting ten days ago, INR 7.8 today.

Other contributors to check before blaming the drug alone: missed or doubled doses, alcohol, NSAIDs or aspirin bought over the counter, acute illness, liver function, thyroid status, and dietary vitamin K.

Risk stratification:

• Bleeding risk is currently low - no bleeding, stable Hb, normal blood pressure, no recent surgery.
• Thrombotic risk is high if he is left unprotected - a mechanical mitral valve thromboses quickly, and under-anticoagulation is as dangerous for him as over-anticoagulation.

So this is a high-INR, low-bleeding-risk, high-thrombotic-risk situation, which dictates a measured correction rather than a dramatic one.

• Omit warfarin for one to two doses. Recheck the INR in 24–48 hours, then every 2–3 days until it is back in range.
• Vitamin K: with no bleeding, simply withholding warfarin is acceptable. If his bleeding risk is judged high (recent surgery, thrombocytopenia, uncontrolled hypertension, falls), give phytomenadione 1–2.5 mg orally. Oral, not intramuscular - IM injection causes haematoma in an anticoagulated patient. Do not give 10 mg IV, which produces prolonged warfarin resistance and risks valve thrombosis.
• Do not give fresh frozen plasma or prothrombin complex concentrate. These are for major bleeding or urgent surgery, not for an asymptomatic high INR; they carry volume and thrombotic risk.
• Do not stop the antibiotic and walk away - either confirm the UTI has resolved or switch to an agent with less CYP2C9 interaction, for example nitrofurantoin if renal function permits.
• Monitor for bleeding: neurological observations, urine and stool inspection, Hb at 48 hours.
• Safety-netting: issue a written warning card listing warfarin interactions, advise paracetamol rather than NSAIDs for pain, and give him a direct telephone number because he lives 30 km away and relies on his wife for transport.

Restart: when the INR falls below the top of his target range (below 3.5, ideally near 3.0), restart at a reduced dose - for example 4 mg alternating with 3 mg rather than 5 mg daily - and recheck in 3–5 days, then weekly for two to three weeks. Never leave a mechanical mitral valve unprotected for more than 48–72 hours without a bridging plan.

Dental extraction in three days: an INR of 7.8 is an absolute reason to postpone a planned extraction. Recheck the day before the appointment. If the INR is below 3.5 (some guidelines accept up to 4.0), a single simple extraction can proceed without stopping warfarin, using local pressure and tranexamic acid 4.8% mouthwash, with the dentist aware of the anticoagulant.

Do not bridge with low molecular weight heparin for a single dental extraction. Bridging is reserved for high-thrombotic-risk valves when warfarin must be interrupted for more than 48–72 hours; here the bleeding risk of bridging outweighs the benefit and the extraction can be done without interruption.

Coordinate the dentist, the physician and the patient in one written plan, and reschedule rather than improvise if the INR is still high on the day.