INR 6.2 After a Course of Trimethoprim–Sulfamethoxazole
A 74-year-old woman with atrial fibrillation has taken warfarin 4 mg daily for 3 years with stable INRs between 2.1 and 2.8 (target 2.0–3.0). Four days ago her family physician started trimethoprim–sulfamethoxazole DS twice daily for 7 days for dysuria. Today she attends the pharmacy for a routine check: INR 6.2. She feels well, has no bruising, no haematuria, no melena, and no epistaxis; BP 138/76, pulse 74/min and irregular. Other medications: metoprolol, ramipril, atorvastatin, and occasional acetaminophen. She ate the same diet as usual, has had no alcohol, and denies missed or extra warfarin doses.
1. Explain the mechanism of this interaction and identify the additional contributing factors you would screen for.
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- Pharmacodynamic/dynamic mechanism: trimethoprim–sulfamethoxazole is a potent inhibitor of CYP2C9, the main enzyme metabolizing S-warfarin (the more active enantiomer). Inhibition reduces warfarin clearance, so the INR rises even though the dose is unchanged. The effect typically peaks within 3–7 days.
- Second mechanism: sulfamethoxazole is highly protein-bound and can displace warfarin from albumin, transiently increasing the free (active) fraction. It also suppresses gut flora that produce vitamin K, adding a third, smaller effect.
- Acute illness itself contributes: reduced oral intake, fever, and reduced vitamin K production all raise INR.
- Screen for other causes before attributing everything to the antibiotic: confirm the dose and adherence, ask about acetaminophen (doses >2 g/day can raise INR), any new OTC product (NSAIDs, ASA, cough preparations containing guaifenesin or possibly vitamin E), alcohol, change in leafy green intake, diarrhea, thyroid status, and liver function.
- No bleeding - her only risk marker is a supratherapeutic INR. Check a CBC and creatinine, and inspect for occult bleeding.
2. State exactly what you would recommend for the warfarin and vitamin K now, and justify why no vitamin K is the right default in this patient.
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- Withhold warfarin for 1–2 doses and reduce the weekly dose when restarting (a reasonable approach is a 10–20% dose reduction, guided by the next INR).
- Do not give vitamin K by default. Guidance is:
- INR above target but <10 with **no bleeding** -> omit warfarin, no vitamin K (oral vitamin K 1–2.5 mg may be considered if there are additional bleeding risk factors, e.g., recent surgery, platelet dysfunction, prior GI bleed, or a falling haemoglobin).
- INR 10 or above without bleeding -> oral vitamin K 2.5–5 mg.
- Any major bleeding -> stop warfarin, give vitamin K 5–10 mg IV plus prothrombin complex concentrate.
- Why no vitamin K here: she is asymptomatic, so the risk of anaphylactoid reaction and of making her warfarin-resistant for days (putting her at risk of a stroke from subtherapeutic anticoagulation) outweighs the benefit. Her AF means an over-correction is not a neutral error.
- Monitoring: recheck INR in 24–48 hours, then every 2–3 days until it is back in range, because the CYP2C9 inhibition persists for several days after the antibiotic course ends. Watch for a rebound below therapeutic range once the antibiotic clears.
3. The antibiotic still has 3 days to run. What is your recommendation for treating the UTI, and what else would you do at this visit?
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- Change the antibiotic if clinically appropriate. TMP-SMX is a strong CYP2C9 inhibitor with a well-documented interaction, and there are alternatives for uncomplicated cystitis: nitrofurantoin (avoid if CrCl is low - below about 30 mL/min - and it lacks reliable tissue penetration for pyelonephritis), fosfomycin single dose, or pivmecillinam where available. Confirm the organism and susceptibilities if a culture was sent, and confirm she does not have pyelonephritis (fever, flank pain, systemic symptoms), which would need a different agent and duration.
- If the antibiotic must continue (e.g., culture-directed necessity), reduce the warfarin dose now, check INR in 2–3 days, and counsel her on bleeding precautions.
- At this visit:
- Assess for bleeding (skin, urine, stool, neurological symptoms).
- Arrange the 24–48 hour INR recheck and give her the result plan in writing.
- Provide a clear written warfarin plan: which doses to hold, the new schedule, and the next blood test date.
- Counsel on maintaining consistent vitamin K intake, limiting alcohol, checking with the pharmacist before any OTC or new prescription, and using acetaminophen at the lowest effective dose.
- Flag the interaction in the pharmacy record so future antibiotic prescriptions trigger a warning, and consider whether a DOAC would be preferable if these episodes recur.
- Do not simply tell her to stop warfarin - stroke risk in AF is the reason she is anticoagulated.