The relationship between maintenance dosages of three vitamin K antagonists: acenocoumarol, warfarin and phenprocoumon.

INTRODUCTION Vitamin K antagonists of the coumarin type are widely used oral anticoagulants. OBJECTIVE We developed a transition algorithm for the maintenance dosages of three frequently used coumarins: warfarin, phenprocoumon and acenocoumarol. METHODS The study was conducted at the Leiden Anticoagulation Clinic. Patients were participants in a trial of which the main objective was to compare the quality of an oral anticoagulant therapy with phenprocoumon to warfarin. We included patients who initiated oral anticoagulant therapy and patients who were already using acenocoumarol. Patients were randomized to a treatment with warfarin or phenprocoumon. Patients who were randomized to warfarin switched to phenprocoumon at the end of follow-up. We analysed the switch from acenocoumarol to warfarin or phenprocoumon at the start of follow-up and the switch of warfarin to phenprocoumon at the end of follow-up and calculated the transition factors for stable anticoagulation between these three vitamin K antagonists. RESULTS Fifty-eight patients switched from warfarin to phenprocoumon, 39 from acenocoumarol to phenprocoumon and 44 from acenocoumarol to warfarin. The maintenance dose of warfarin was 0.41 (95%CI 0.39-0.43) times the maintenance dose of phenprocoumon. The transition factor between acenocoumarol and phenprocoumon was 0.84 (95%CI 0.79-0.89) and between acenocoumarol and warfarin 1.85 (95%CI 1.78-1.92). CONCLUSIONS We determined the transition factors between warfarin, phenprocoumon and acenocoumarol. With these transition factors physicians are able to estimate the maintenance dose when it is necessary for a patient to switch from one coumarin to the other.