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Warfarin-Antibiotic Interaction in IE — SCE Infectious Diseases MCQ

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HardAntimicrobial StewardshipWarfarin-Antibiotic Interaction in IESCE Infectious Diseases

A 55-year-old man with a mechanical mitral valve develops Enterococcus faecalis prosthetic valve endocarditis. He is on Warfarin. During treatment with Ampicillin plus Gentamicin, his INR becomes extremely difficult to control. What pharmacological factor explains this?

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Correct answer: D — IV Ampicillin (and other penicillins) can reduce Warfarin efficacy by disrupting vitamin K-producing gut flora AND can interact with Warfarin through altered protein binding — frequent INR monitoring is needed

Multiple factors affect INR stability during endocarditis treatment: (1) IV antibiotics (particularly penicillins and cephalosporins) disrupt intestinal flora that produce vitamin K, potentially increasing INR; (2) sepsis/inflammation affects hepatic CYP metabolism of Warfarin; (3) dietary changes during hospitalisation; (4) drug interactions (Rifampicin if used — potent CYP inducer reducing Warfarin levels; Metronidazole if used — CYP2C9 inhibitor increasing levels). Very frequent INR monitoring (initially daily, then 2–3 times weekly) is essential. Warfarin should NOT be stopped in mechanical valve patients.

Reference: ESC 2023 – Endocarditis; BNF 2024 – Warfarin interactions