BACKGROUND: Clinical guidelines recommend oral anticoagulation for stroke 
prevention in patients with atrial fibrillation (AF) at moderate or high risk 
for stroke but not at high risk for bleeding; however, studies consistently 
report suboptimal use of such therapy. This study used Medicare Part D claims 
data to assess the use of warfarin in the Medicare population.
OBJECTIVES: To compare real-world warfarin utilization with current treatment 
guideline recommendations, and to assess the effect of warfarin exposure level 
on patient outcomes in Medicare beneficiaries with nonvalvular AF (NVAF).
METHODS: Patients who were recently diagnosed with NVAF were identified using a 
random 5% sample of Research Identifiable Files of Medicare beneficiaries in 
2006 or 2007. Individuals with moderate-to-high stroke risk per CHADS2 but not 
at high bleeding risk per ATRIA (Anticoagulation and Risk Factors in Atrial 
Fibrillation) bleeding risk score were evaluated for warfarin use, as identified 
by the presence of ≥1 warfarin prescription claims within 12 months after the 
index diagnosis. Warfarin exposure level was assessed by the proportion of days 
covered during the 12-month follow-up period. The effect of warfarin exposure on 
ischemic stroke and major bleeding event rates during the 12-month follow-up 
period were assessed using multivariate logistic regression.
RESULTS: Data from 14,149 newly diagnosed patients with NVAF (mean age, 79 
years; 58.7% female) were analyzed, and of these, 7524 (53.2%) patients were 
identified as having moderate-to-high stroke risk and not being at high bleeding 
risk. Of these patients, 3110 (41.3%) did not receive warfarin within 12 months 
of the index diagnosis. The risk for ischemic stroke was significantly lower in 
those with warfarin exposure versus no warfarin exposure (adjusted odds ratio 
[OR], 0.51; confidence interval [CI], 0.43-0.61; P <.001) and in patients with 
warfarin proportion of days covered ≥80% versus those with proportion of days 
covered <80% (adjusted OR, 0.59; 95% CI, 0.48-0.72; P<.001). Warfarin exposure 
was associated with a significantly higher major bleeding rate (adjusted OR, 
1.19; 95% CI, 1.04-1.36; P = .013), with this significant difference being 
driven by patients aged >65 years.
CONCLUSIONS: Based on a risk-stratification scheme composed of previously 
published tools, such as CHADS2 and the ATRIA bleeding risk index, a significant 
proportion of Medicare beneficiaries with AF are not receiving 
guideline-recommended anticoagulation therapy, which leads to an excess rate of 
ischemic stroke in this patient population. These findings highlight 
quality-of-care issues for patients with AF and the need to improve compliance 
with anticoagulation guidelines in the Medicare population.