Case report of a patient with acenocoumarol resistance following therapy for infective endocarditis with rifampicin and gentamicin combination therapy.

Vitamin K antagonists (VKAs) are essential for anticoagulation in patients with mechanical heart valves, but are affected by drug interactions, particularly with rifampicin - a potent inducer of CYP2C9 that accelerates VKA metabolism and destabilises INR levels. A 48-year-old male with multiple aortic valve replacements and infective endocarditis required rifampicin therapy post-surgery. Despite escalating doses of warfarin and acenocoumarol, therapeutic INR levels were not achieved, requiring low molecular weight heparin (LMWH) bridging. INR stabilisation occurred only after discontinuing rifampicin, with a drastic reduction in VKA dose requirements. Managing anticoagulation in patients on VKAs and rifampicin remains challenging due to rapid metabolism and INR fluctuations. Close INR monitoring, dose adjustments and alternative strategies, such as twice-daily VKA dosing or LMWH bridging, are crucial for maintaining therapeutic anticoagulation.