Background: Warfarin-related bleeding occurred most commonly during the first three months of therapy, possibly due to an initiating overdose of warfarin. Previous small study reported that 3-mg initiating dose of warfarin appeared to be safe in Thai patients.

Objective: To compare the performance of simplified warfarin dosing formula and 3-mg initiating dose to predict actual warfarin dose that achieved therapeutic range of target international normalized ratio (INR).

Materials and Methods: The present study was a retrospective study including 640 patients who had been receiving warfarin with target INR of 2.0 to 3.0. The actual warfarin dose was defined as warfarin dose that resulted in INR of 2.0 to 3.0 for at least two consecutive follow-ups after initiation. The simplified warfarin dosing formula was 3.2 – (0.03×age(years)) + (0.02×body weight(kg)) (10% dose reduction if presence of heart failure (HF) and/or stroke). The optimal dosage was defined as difference from actual dose as being within 20%.

Results: Mean age was 65±13 years. The mean actual dose of warfarin was 2.8±1.2 mg. The warfarin dosing formula resulted in optimal dosing in 41% and overdosing in 21% of cases, whereas 3-mg initiating dose resulted in optimal dosing in 39% and overdosing in 43% of patients. In patients with HF and/or stroke, using formula resulted in overdosing in 23% of cases, whereas 3-mg initiating dose led to overdosing in 53% of patients.

Conclusion: A simplified warfarin dosing formula appeared to be safer than 3-mg initiating dose. Overdosing after using warfarin formula was less prevalent than using 3-mg initiating dose particularly in patients with HF and/or stroke.

Keywords: Warfarin, Algorithms, Atrial fibrillation, Anticoagulant

Received 15 May 2019 | Revised 4 Jul 2019 | Accepted 11 Jul 2019