Decreased warfarin effects in elder with recurrent Clostridium difficile infection during fidaxomicin therapy: a case report

Clostridium difficile infection is a disease with increasing incidence, particularly in high‑riskpatients such as the elderly, immunocompromised patients, etc.We report an unexpected decrease of International Normalized Ratio (INR) response to warfarin during a first recurrence of Clostridium difficile infection (CDI) treated with fidaxomicin. The patient, an old man who has prosthetic heart valves on anticoagulation therapy with warfarin, was treated with an association of vancomycin plus metronidazole for a first episode of CDI. Patient remained symptom‑free for few days and then he presented with recurrent diarrhea. A retreatment with vancomycin and metronidazole didn’t resolve symptoms of CDI, therefore he underwent fidaxomicin treatment for 10 days, with rapid resolution of diarrhea. In the meantime, warfarin effects diminished, and only with increases of dosage INR therapeutic range was achieved few days after discontinuing fidaxomicin. According to product information, fidaxomicin doesn’t interfere with warfarin. The authors highlight the different plausible mechanisms to explain the association between the unexpected decreased effect of warfarin and factors that could have influenced such event. The frequent update of product information through good pharmacovigilance practices could help clinicians in the management of unexpected events.