Age and first INR after initiation of oral anticoagulant therapy with acenocoumarol predict the maintenance dosage

Background: This retrospective study was performed to develop a model to predict the maintenance dosage of the vitamin K antagonist acenocoumarol, based upon the first INR after a standard initial dosage regimen. Material and Methods: Outpatients with atrial fibrillation (n = 284) and initial regimens of 6-4 or 6-4-2 mg acenocoumarol on day 1, 2 and 3, respectively, were included. The maintenance dosage of the period 3 - 6 months after the instalment was related to the first INR after those standard initial dosage regimens, because in that period the INR was 76% of the time within the therapeutic range and therefore considered suitable to perform the analysis. Results: A clear relation was found between the first INR, the maintenance dosage and the age. A model that predicts the maintenance dosage immediately after the standard initial dosage and based on the first INR and adjusted for age, has been developed, according to the formula: required dosage = 5.03-1.65 * ln (first INR) - 0.01 * age. Conclusion: We have developed a formula to predict the maintenance dosage of acenocoumarol. With this formula it is possible to install this maintenance dosage and thus achieve oral anticoagulant therapy within the therapeutic range at an earlier stage. This will have to be shown in a prospective study.