Safe introduction of warfarin for thrombotic prophylaxis in atrial fibrillation requiring only a weekly INR

A new protocol for initiating warfarin therapy was introduced to reduce the workload in the Anticoagulant Clinic. A total of 200 outpatients, with a median age of 74 years, requiring anticoagulation for atrial fibrillation, commenced warfarin 3 mg daily for 1 week. Patients were initially seen weekly, and subsequent warfarin doses were dictated by the International Normalized Ratio (INR) on days 8 and 15; 84% of patients followed the protocol correctly: of these 86% had an INR > or =2 by day 15 and >98% had INR >2 by day 22. By day 22, 58% of patients achieved a stable dose, 85% by day 29 and >95% by day 36. Day 8 INR was predictive of the final maintenance dose required. No patient suffered any thrombotic or haemorrhagic complications in the first month: only three patients had an INR >3 on day 8, and 11 patients had an INR >4 on day 15. Patient age and sex were not sufficiently related to warfarin requirement to provide useful predictive information. This protocol, requiring only weekly INRs, has proved safe and effective for outpatient warfarinization, and has reduced clinic attendances in this population.