Evaluation of an initiation protocol of 4 mg of warfarin for atrial fibrillation in the outpatient setting.

OBJECTIVE: To describe the efficacy and safety of an initiation algorithm for 4 mg of warfarin in ambulatory patients with atrial fibrillation. DESIGN: Prospectively planned retrospective chart review. SETTING: Centre for Family Medicine Family Health Team in Kitchener, Ont. PARTICIPANTS: Ambulatory patients requiring anticoagulation for atrial fibrillation. INTERVENTIONS: Patients were prescribed 4 mg of warfarin to be taken once daily for 3 days. An international normalized ratio (INR) measured on the morning of the fourth day was used to predict the maintenance dose of warfarin. Subsequent INR measurements were obtained biweekly until patients reached their actual maintenance dose. MAIN OUTCOME MEASURES: Number of INR values greater than or equal to 4.0 before the warfarin maintenance dose was achieved. Secondary outcome measures included thromboembolic and bleeding events, number of days required to reach therapeutic INR, and correlation between predicted and actual warfarin maintenance dose. RESULTS: Twenty-five patients were included in the study. The average age was 76.0 years (range 56.0 to 89.0), and 17 patients were women. The average CHADS2 (congestive heart failure, hypertension, age ≥ 75 years,diabetes mellitus, and stroke or transient ischemic attack) score was 2.0.Only 1 patient had an INR greater than 4.0 during the study period. The mean time to achieve a therapeutic INR was 11.0 days. The day 4 INR was moderately predictive of the maintenance dose (r2 = 0.47). There were no adverse events that required medical attention during the study period. CONCLUSION: In this pilot study, an initiation algorithm for 4 mg of warfarin was safe and achieved a therapeutic INR within a reasonable time frame in outpatients with atrial fibrillation.