Objective To examine whether warfarin use and outcomes differ across CHADS₂ and CHA₂DS₂-VASc risk strata for non-valvular atrial fibrillation (NVAF).

Design Population-based cohort study using linked administrative databases in Alberta, Canada.

Setting Inpatient and outpatient.

Patients 42 834 consecutive patients ≥20 years of age with newly diagnosed NVAF.

Main outcome measures Cerebrovascular events and/or mortality in the first year after diagnosis.

Results Of 42 834 NVAF patients, 22.7% were low risk on the CHADS₂ risk score (0), 27.5% were intermediate risk (1), and 49.8% were high risk (≥2). The CHA₂DS₂-VASc risk score reclassified 16 722 patients such that 7.8% were defined low risk, 13.8% intermediate risk and 78.4% high risk. Of the elderly cohort (≥65 years) with definite NVAF visits (at least two encounters 30 days apart, n=8780), 49% were taking warfarin within 90 days of diagnosis. Warfarin use did not differ across risk strata using either the CHADS₂ (p for trend=0.85) or CHA₂DS₂-VASC (p=0.35). In multivariable adjusted analyses, warfarin use was associated with substantially lower rates of death or cerebrovascular events for patients with CHADS₂ scores of 1 (OR 0.52, 95% CI 0.41 to 0.67) or ≥2 (OR 0.61, 95% CI 0.53 to 0.71), or CHA₂DS₂-VASc scores of ≥2 (OR 0.60, 95% CI 0.53 to 0.68).

Conclusions In elderly patients with NVAF and elevated CHADS₂ or CHA₂DS₂-VASC scores, warfarin users exhibited lower rates of cerebrovascular events and mortality. However, warfarin use did not differ across risk strata, another example of the risk–treatment paradox in cardiovascular disease.