Evaluation of a simple dosage scheme for transition from phenprocoumon to warfarin in oral anticoagulation

Phenprocoumon, whose elimination half-time is 144 hours, has been the tradi tional oral anticoagulant of choice in Europe, However, today's most widely used drug is warfarin, whose elimination half-time is 40 hours. This study aims to evaluate a method for safe transition from phenprocoumon to warfar in, which is sometimes required. Hence, the large difference in their elimi nation rates may on occasion lead to serious overdosage upon transition fro m one drug to the other. According to average equipotent doses, a stepwise increase in warfarin dose was calculated based on the elimination half-time s of the two drugs. The dosage scheme was subsequently tested in a pilot st udy including 35 patients. The conversion scheme was then adjusted based on the results from the pilot study. The new scheme was tested in 69 patients , The transition factor was 2.3, which implies that equipotency was achieve d when the warfarin dose was 2.3 times larger than the phenprocoumon dose ( in mg). This scheme proved optimal for 75% of the patients, However, the do se had to be adjusted individually in the remaining 25% of the patients to a level corresponding to the measured international normalised ratios. No p atients experienced haemorrhages or thromboembolic complications during the period of changeover. In conclusion, the proposed scheme for changing medi cation from phenprocoumon to warfarin is safe and convenient. (C) 2000 Else vier Science Ltd. All rights reserved.