Significant increase in warfarin (Coumadin) therapy in older patients can be credited to a growing prevalence of atrial fibrillation. Patients with atrial fibrillation are at increased risk of cerebrovascular accidents and death, but the use of warfarin therapy can reduce this risk significantly. Older patients have an enhanced dose response to warfarin and are more likely to become overanticoagulated. They also take longer to return to a safe International Normalized Ratio (INR) level than younger patients.

There are no established guidelines for initiating warfarin therapy in the very elderly (80 years and older). The recommended starting warfarin dosage of 5 mg per day could result in INR levels in the very elderly that are above the goal range of 2.0 to 3.0. Garcia and colleagues evaluated the effect of age, sex, and indication on warfarin maintenance dosing in ambulatory adults.

The first part of the study was a prospective cohort involving participants from multiple sites enrolled from August 2000 through February 2002. Patients were included if they were 18 years or older, had a physician-specific INR of 2.0 to 3.0, and had achieved a stable warfarin dose. The data from each site transmitted to an independent databank included information about age, sex, ethnicity, reason for anticoagulant therapy, physician-specific INR range, INR values and dates of testing, warfarin dose at each visit, and the presence of other comorbidities. The second part of the study was a retrospective cohort with participants from one site conducted over 10 years. Information on age, sex, indication for oral anticoagulant therapy, INR values, dates of testing, and warfarin dosing was collected.

A total of 4,616 patients from 101 community-based sites were enrolled in the prospective study, and a total of 7,586 patients from one site enrolled in the retrospective study. The total patient population in both studies was 12,202 including 2,359 patients 80 years and older. The median weekly warfarin dose for men younger than 50 years was 45 mg (6.4 mg per day), whereas in women 80 years and older, the median weekly dose was 22 mg (3.1 mg per day). During the study, the weekly dose declined by 0.4 mg per additional year of age. Women required 4.5 mg of warfarin less per week than men. If patients older than 70 years were started on the recommended dosage of 5 mg per day of warfarin, 82 percent of the women and 65 percent of the men would be overanticoagulated.

The authors conclude that warfarin dose requirements decline greatly with age, and older women require the lowest dose. They add that the practice of starting warfarin therapy at 5 mg per day would result in overanticoagulating a significant number of older patients. They recommend that lower initiation and maintenance doses should be considered in this age group.