Voriconazole-warfarin interaction necessitating warfarin dose management in an invasive aspergillosis patient: A case report

Purpose Voriconazole has been shown to interfere with the anticoagulant efficacy of warfarin. The package insert warns of the risk of prolonged bleeding times when prescribing the drug. We report a case that resulted in the successful management of this interaction by warfarin dose adjustment during the treatment of invasive aspergillosis. Methods The case involved a 61-year-old Korean female previously treated with warfarin on a stable weekly dose of 24-28 mg. Her genetic polymorphisms included VKORC1-1639G >A GA, CYP2C9*3 42614A> C AA and CYP4F2*3 18000G> A GG. The patient was diagnosed with invasive aspergillosis and treated with voriconazole. She was subsequently hospitalized with a prolongation of prothrombin (PT) time for which the highest international normalized ratio (INR) value reached 7.46. After an interaction between voriconazole and warfarin was suspected, the warfarin dose was reduced to 9 to 21 mg/week. The patient was stabilized on a dose of 10.5 mg/week, six weeks post-voriconazole therapy and her INR value was 2.30. Conclusion This case highlights the significance of the voriconazole and warfarin interaction, and suggests that warfarin may require an approximate 13-68% reduction when co-administered with voriconazole.