Predicting warfarin maintenance dosage based on initial response

The relationship between initial response to warfarin sodium, as measured by prothrombin time, and warfarin maintenance dose was studied. Hospital patients received 10 mg of warfarin sodium daily until the prothrombin ratio (patient's prothrombin time/control) was 1.5 to 2.5. Patients subsequently were maintained at that warfarin dosage which maintained prothrombin ratios within this range. Twenty-two patients completed the study. A correlation was found between maintenance dose and the logarithm of the prothrombin ratio on the third day after initiating warfarin therapy (p less than 0.01, r = -0.74). There was no relationship between maintenance dose and the number of days needed to reach therapeutic response. Patients with baseline prothrombin ratios of less than one took significantly longer to reach therapeutic response than did patients with baselines of greater than one (p less than 0.01y. A simple yet scientific approach to determining warfarin maintenance dosage would seem to be feasible. The correlation found between maintenance dose and prothrombin ratio in this study would have to be applied cautiously and only after further validation.