Appropriate International Normalized Ratio Follow-up Interval in Patients with Atrial Fibrillation Taking Warfarin

Background: The international normalized ratio (INR) is recommended to be followed up every four-week intervals in patients with atrial fibrillation (AF) taking warfarin. However, data about the appropriate INR follow-up interval are lacking in those patients. Objective: We aimed to investigate the appropriate INR follow-up intervals in AF patients receiving warfarin between 6-week and 12-week intervals of follow-up appointments. Methods: We retrospectively enrolled patients with AF taking warfarin at the Central Chest Institute of Thailand between January 2017 to May 2023. The primary outcome was time in therapeutic range (TTR) in each group of follow-up interval and secondary outcome was a composite outcome of acute ischemic stroke/transient ischemic attack (TIA)/systemic embolic events, major bleeding, minor bleeding, and/or all-cause mortality between 6-week and 12-week groups. Results: A total of 400 AF patients taking were recruited in our study. The average CHA2DS2-VASc score was 3.6 and 3.7 in 6-week and 12-week groups, respectively. The average HAS-BLED score was 1.8 in both groups. The TTR in 12-week group was not inferior to 6-week group (absolute difference 1.5 percentage points; 95% confidence interval -2.4 to 5.3; P < 0.001 for non-inferiority). In addition, rate of acute ischemic stroke/TIA/systemic embolic events, major bleeding, minor bleeding, and/or all-cause mortality was not statistically significant between patients in 12-week and 6-week groups. Conclusion: The TTR in 12-week group was not inferior to 6-week group in patients with AF taking warfarin. Extended 12-week follow-up interval appeared to be appropriate and safe in clinical practice.