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Ampicillin-Warfarin IE — SCE Infectious Diseases MCQ

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ModerateAntimicrobial StewardshipAmpicillin-Warfarin IESCE Infectious Diseases

A 60-year-old man with a metallic mitral valve on Warfarin develops E. faecalis prosthetic valve endocarditis. His team plans Ampicillin plus Ceftriaxone (double beta-lactam for HLAR). He is also on Warfarin. What specific drug interaction with Ampicillin affects Warfarin?

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Correct answer: A — High-dose IV Ampicillin can disrupt vitamin K-producing gut flora, potentially increasing INR — very frequent INR monitoring (initially daily, then 2–3 times weekly) is needed

IV Ampicillin at endocarditis doses (2 g 4-hourly) disrupts intestinal flora that produce vitamin K, potentially increasing Warfarin effect and INR. Additionally, acute illness (sepsis, reduced oral intake) affects Warfarin pharmacokinetics. Very frequent INR monitoring is essential during endocarditis treatment. The double beta-lactam regimen (Ampicillin plus Ceftriaxone) for HLAR enterococcal IE doubles the beta-lactam-Warfarin interaction risk.

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