Introduction: Concomitant administration of rifampicin and warfarin poses a challenge in achieving stable therapeutic international normalised ratio (INR). There is no published case report in Singapore to show the time course for enzyme induction and de-induction in Asian patient.

Case Report: A 90-year-old man was on a stable warfarin dose of 1.5 mg daily for stroke prevention in atrial fibrillation with therapeutic INR before switching to rivaroxaban and subsequently apixaban. He was initiated with a 9-month course of rifampicin, isoniazid, pyridoxine and ethambutol for treatment of pulmonary tuberculosis. Hence, apixaban was switched back to warfarin. Therapeutic INR was first achieved 20 days after a dose increment of 166% in warfarin from 1.5 mg daily to 4 mg daily. The INR subsequently decreased again and warfarin dose was increased to 4.5 mg daily. A 2-fold increment in warfarin dose was required to reach the second therapeutic INR at week-7. INR was stable in therapeutic range with 4.5 mg daily and patient was followed up every 2 to 4 weeks. Time in therapeutic range (TTR) was 74% over the course of nine months. After five days of discontinuing rifampicin, INR decreased to 1.70 despite maintaining the same dose of warfarin.

Conclusion: The total time course of enzyme induction takes about six to seven weeks to reach a steady state. The de-induction of enzyme may not occur within the first week of rifampicin discontinuation. Close INR monitoring during the initial phase of initiation or termination of rifampicin is required.

Keywords: Enzyme induction, Interaction, Rifampicin, Time Course, Warfarin