Abstract 5506: Warfarin Effectiveness and Stroke Rate in Ambulatory Patients With Non-valvular Atrial Fibrillation Managed in Community-based Primary Care Practices
Background: Despite the known efficacy of warfarin for the prevention of stroke in non-valvular atrial fibrillation (NVAF), its effectiveness in primary care practice is less clear. This study examined the use of warfarin, the time in therapeutic range, and the incidence rate of first-time stroke in NVAF patients in a community-based primary care population.
Methods: We performed a retrospective evaluation of patients with NVAF from 1998–2009 in a large health system with a shared electronic medical record. The presence of NVAF, warfarin use, INR values, and stroke events were collected through a review of office and hospital records, laboratory values, and medication use. Exposure was categorized into four categories: no warfarin, warfarin with INR < 2, warfarin with INR 2–3, and warfarin with INR > 3. Person-time in days was computed for each category and incidence rates for stroke by warfarin exposure were computed.
Results: A total of 840 NVAF patients qualified; with a mean age of 71 years, 49% female, and 14% African-American. Of these, 511 were exposed to warfarin, of whom 207 (40.5%) had complete INR records available. During 885 person-years of follow-up for these 207 patients, warfarin was continued through only 48.9% (432 person-yrs.). The INR was maintained at or above therapeutic range for 60.3% (260 person-yrs.) of the time for patients on warfarin. There were 18 strokes in the 207 with complete INR values, of which 11 (61%) occurred while off anticoagulation. Effective anticoagulation was associated with lower stroke rates (see table⇓).
Conclusion: In this community-based setting, of the patients with complete INR records, effective warfarin use was associated with a 37.5% reduction in stroke risk; however, in these patients anticoagulation was attempted less than 50% of the time and therapeutic range achieved less than 25% of this time. Our data suggest further inquiry is needed into the reasons for failing to achieve adequate anticoagulation in actual practice.