Warfarin dosage requirements: prospective clinical trial of a method for prediction from the response to a single dose

We have previously described a model for predicting individual daily maintenance dosage (MD) requirements of warfarin 24 h after the administration of a single dose. This model relies on measurement of the initial anticoagulant response as the 24 h percentage fall in plasma clotting factor VII activity. It permits prediction of the individual MD given the size of the initial dose, a baseline and desired maintenance value of the prothrombin ratio, and a baseline and 24 h plasma level of factor VII activity. We now present the results of a prospective clinical trial of the method. Data from 65 patients were suitable for analysis. The mean daily MD of warfarin was 4.0 mg (range 1-10 mg). There was a moderately strong linear relationship between predicted and actual MDs of warfarin (r = 0.66, P less than 0.001). Actual vs predicted MDs in individual patients were not significantly different. The mean difference was 0.39 mg. The results of this prospective trial suggest that our model predicts warfarin MD requirements with reasonable accuracy. Nevertheless, the accuracy of the model is not sufficient to replace careful clinical and haematological monitoring of each patient commencing warfarin therapy.