Material and Method: This retrospective study enrolled 230 AF patients that received warfarin in Siriraj Hospital between January 1, 2005 and December 31, 2009 and collected the lNR level at the time of the event, the numbers of ischemic stroke, and bleeding events. The incidence density of ischemic stroke or bleeding events was calculated by dividing the number of ischemic stroke or bleeding event in each INR level with the summation of the time that each patient stayed in each INR group. The patients with a prosthetic valve were excluded. The INR range was classified into six groups (less than 1.5, 1.5 to 1.9, 2.0 to 2.4, 2.5 to 2.9, 3.0 to 3.4, and greater than 3.4). The optimal INR level was defined as the lowest incidence density of ischemic stroke and bleeding complications.

Results: Two hundred thirty AF patients (the mean age 68±12 years) were enrolled, contributing to 737.54 patient-years of observation period. Of the 230 patients, nine patients experienced 12 ischemic events (1.6 per 100 patient-years) and 54 patients experienced 57 bleeding events (7.7 per 100 patient-years). The percentage of patient-time spent within INR 2 to 3, INR less than 2, and INR more than 3 were 40.75, 46.22, and 13.03%, respectively. The INR level more than 3.4 increased both major and minor bleeding events (p = 0.001). The INR level of 3.0 to 3.4 increased the minor bleeding events (p = 0.03). The INR level less than 1.5 increased incidence of ischemic stroke (p = 0.03). The overall event rate was lowest in the INR range from 1.5 to 2.9, which is significantly different from that of INR more than 2.9 (p<0.0001), but trend lower than INR less than 1.5 without being statistically significant (p = 0.198).

Conclusion: An INR of 1.5-2.9 appeared to be associated with the lowest incidence rate of bleeding or ischemic stroke in a cohort of Thai AF patients receiving warfarin therapy for stroke prevention.

Keywords: International normalized ratio (INR), Warfarin, Anticoagulation, Atrial fibrillation, Ischemic stroke, bleeding