A simple formula for predicting the warfarin dose in atrial fibrillation: development, external validation, and model comparison

Abstract

Background: The dose of warfarin varies between individuals. Several formulas for predicting the maintenance dose of warfarin have been developed; however, most are complicated and not practical for clinical use. Objective: To determine factors that predict warfarin dosage and the relationship between clinical variables and the maintenance dose of warfarin, and to develop a simple formula for predicting the maintenance dose of warfarin that is particularly useful for identifying patients with atrial fibrillation (AF) who are at higher risk of bleeding, without relying on pharmacogenetic data. Materials and methods: This was a retrospective cohort study carried out between 2011 and 2021. All patients are started on warfarin with a target INR of 2.0 to 3.0. The prediction models for the maintenance dose were developed using a first-order equation. Correlation and performance of the formula were examined in training and validation cohorts. Results: A training cohort consisted of 520 patients with a mean age of 69 ± 12 years. The proposed warfarin dosing formula was 3+(0.02×body weight (kg))-(0.02×age (years))-(0.4×serum creatinine (mg/dL)).When compared with a warfarin dosing formula, a 3-mg dose was associated with overdosing with an odds ratio [OR] of 3.31 (95%CI 2.26–4.84, p < 0.0001) in patients whose body weight was < 60 kg, OR 3.08 (95%CI 2.15–4.40, p < 0.0001) in patients aged ≥ 70 years and OR 2.39 (95% CI 1.67–3.44, p < 0.0001) in patients with eGFR < 50 mL/min. The findings in the validation cohort of 632 patients were concordant with the training cohort. Conclusion: A simple warfarin dosing formula incorporating age, body weight, and serum creatinine reduced the risk of warfarin overdose in a high-risk population.