Initiation of warfarin therapy in elderly medical inpatients: A safe and accurate regimen

Purpose Elderly patients are at high risk of over-anticoagulation when treated with warfarin, especially during treatment induction. We developed a simple low-dose regimen for starting warfarin therapy in elderly inpatients. The daily maintenance dosage is predicted from the international normalized ratio (INR) measured the day after the third daily intake of a 4-mg dose. We conducted a prospective multicenter study to evaluate the accuracy and safety of this regimen. Methods We studied 106 elderly (age ≥70 years) inpatients (mean [± SD] age, 85 ± 6 years; range, 71 to 97 years) who had a target INR of 2.0 to 3.0. Accuracy in predicting the daily maintenance dose from INR value on day 3 was evaluated. Results The predicted daily maintenance warfarin dose (3.1 ± 1.6 mg/d) correlated closely with the actual maintenance dose (3.2 ± 1.7 mg/d; R2 = 0.84). The predicted dose was equal to the actual dose in 77 patients (73%; 95% confidence interval [CI]: 64% to 81%) and within 1 mg in 101 patients (95%; 95% CI: 91% to 99%). The mean time needed to achieve a therapeutic INR was 6.7 ± 3.3 days (median, 6.0 days); the mean time needed to achieve the maintenance dose was 9.2 ± 4.5 days (median, 7.0 days). None of the patients had an INR >4.0 during this period. One fatal bleeding event was recorded in a patient with an INR in the therapeutic range. Conclusion Our warfarin induction regimen was simple, safe, and accurate in predicting the daily maintenance warfarin dose in elderly hospitalized patients.