Background: Warfarin is a common drug for thromboembolic prevention in patients with nonvalvular atrial fibrillation (NVAF). However, hemorrhagic complications are also a concern. The optimal international normalized ratio (INR) level is different in each ethnicity.

Objective: To establish the INR range with lowest all adverse event, optimal INR, in Thai NVAF patients from large data, nationwide Warfarin Registry Network (WaRN).

Materials and Methods: The present study was a retrospective cohort study. It analyzed the data of 31,704 Thai NVAF patient between January 1, 2010 and December 31, 2020. The numbers of thromboembolic and bleeding events were correlated to INR level at the time of event. The six groups of INR range were defined as less than 1.5, 1.5 to 1.9, 2.0 to 2.5, 2.6 to 3.0, 3.1 to 3.5, and greater than 3.5.

Results: The data from 797 warfarin clinics were analyzed with an observation period of 21,328.52 patient-years. The median duration of follow-up was 519 days (IQR 172, 1,183). Of the 31,407 patients, 277 patients experienced 319 thromboembolic events, and 3,398 patients experienced 4,860 bleeding events. The thromboembolic events were 4.92, 1.89, 0.86, 0.72, 0.39, 0.36 per 100 patient-years in INR range of less than 1.5, 1.5 to 1.9, 2.0 to 2.5, 2.6 to 3.0, 3.1 to 3.5, and more than 3.5, respectively. The bleeding events were 12.69, 9.51, 8.41, 15.35, 36.38, 125.03 per 100 patient-years in INR range of less than 1.5, 1.5 to 1.9, 2.0 to 2.5, 2.6 to 3.0, 3.1 to 3.5, and more than 3.5, respectively. The all-adverse events were 17.61, 11.40, 9.27, 16.07, 36.77, 125.39 per 100 patient-years in the INR range of less than 1.5, 1.5 to 1.9, 2.0 to 2.5, 2.6 to 3.0, 3.1 to 3.5, and more than 3.5, respectively.

Conclusion: The incidence density of all-adverse events was lowest in INR range of 2.0 to 2.5. This range should be the optimal therapeutic INR level for Thai NVAF patient. However, the INR range can extend to 1.5 to 2.5 in clinical practice.

Keywords: Optimal INR; Nonvalvular Atrial Fibrillation, Bleeding, Thromboembolism

DOI: 10.35755/jmedassocthai.2023.12.13925

Received 12 September 2023 | Revised 7 November 2023 | Accepted 10 November 2023