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

Abstract:To evaluate the safety and cost of anticoagulation therapy in patients newly initiated on warfarin in two institutions in Singapore. All patients newly started on warfarin between December 2011 and May 2012 were recruited and followed up for a period of 6 months. All hospitalization attributed to complications of warfarin therapy, number of INR (international normalized ratio) greater than 4 and total cost of warfarin therapy were collected and analysed. A total of 321 patients were newly initiated on warfarin for standard indications were recruited. At the end of 6 month period, 8.4% of patients were hospitalized for complication related to warfarin therapy. For patients who had stable INR while on warfarin, this rate was almost 2 times lower (3.8% vs 8.4%). In our study group, patients started on warfarin as inpatients had higher rates of bleeding (15.1% vs 6.7%) and more INRs greater than 4 (0.7 per patient vs 0.11 per patient). The cost of initating warfarin was USD660 over the first 6 months of therapy. Patients newly initiated on warfarin were at a heightened risk of bleeding complications as compared to patients already stable on warfarin therapy. This also translates to considerable costs of warfarin initiation.