Warfarin exposure and the risk of thromboembolic and major bleeding events among medicaid patients with atrial fibrillation

BACKGROUND: Outcomes of anticoagulation have been assessed in commercially insured populations, but similar data do not exist for Medicaid populations. OBJECTIVE: To assess the association between warfarin exposure and rates of thromboembolic and bleeding events among patients with nonvalvular atrial fibrillation (NVAF) enrolled in California Medicaid. METHODS: Using a retrospective cohort design based on administrative claims data, we selected Medicaid enrollees aged 50 years and older based on their first claim with a diagnosis of AF between January 1, 1998, and March 31, 2002. Patients were excluded if they had selected contraindications to warfarin, claims for valve replacement procedures, or evidence that AF resulted from transient or reversible causes. Pharmacy claims and prothrombin time tests were used to define subsequent periods of warfarin use (exposure) and nonuse (nonexposure) by all patients. The relative rates of hospitalization for thromboembolic and bleeding events associated with periods of warfarin exposure versus nonexposure were estimated. RESULTS: The 4355 study patients had a mean age of 74 years, and 65% were female. Fifty-nine percent filled any prescriptions for warfarin following AF diagnosis. Across all patients, warfarin exposure occurred during 37% of days after diagnosis. Thromboembolic events were 27% less frequent during periods of warfarin exposure relative to periods of non-exposure (p < 0.01). Major bleeding events were not significantly more common during periods of warfarin exposure (p = 0.55). CONCLUSIONS: In this Medicaid population with NVAF, warfarin use was low and was associated with a relatively modest reduction in thromboembolic events, with no increase in major bleeding risk.