County-level air quality and the prevalence of diagnosed chronic kidney disease in the US Medicare population

BACKGROUND: Considerable geographic variation exists in the prevalence of chronic kidney disease across the United States. While some of this variability can be explained by differences in patient-level risk factors, substantial variability still exists. We hypothesize this may be due to understudied environmental exposures such as air pollution. METHODS: Using data on 1.1 million persons from the 2010 5% Medicare sample and Environmental Protection Agency air-quality measures, we examined the association between county-level particulate matter ≤2.5 μm (PM(2.5)) and the prevalence of diagnosed CKD, based on claims. Modified Poisson regression was used to estimate associations (prevalence ratios [PR]) between county PM(2.5) concentration and individual-level diagnosis of CKD, adjusting for age, sex, race/ethnicity, hypertension, diabetes, and urban/rural status. RESULTS: Prevalence of diagnosed CKD ranged from 0% to 60% by county (median = 16%). As a continuous variable, PM(2.5) concentration shows adjusted PR of diagnosed CKD = 1.03 (95% CI: 1.02–1.05; p<0.001) for an increase of 4 μg/m(3) in PM(2.5). Investigation by quartiles shows an elevated prevalence of diagnosed CKD for mean PM(2.5) levels ≥14 μg/m(3) (highest quartile: PR = 1.05, 95% CI: 1.03–1.07), which is consistent with current ambient air quality standard of 12 μg/m(3), but much lower than the level typically considered healthy for sensitive groups (~40 μg/m(3)). CONCLUSION: A positive association was observed between county-level PM(2.5) concentration and diagnosed CKD. The reliance on CKD diagnostic codes likely identified associations with the most severe CKD cases. These results can be strengthened by exploring laboratory-based diagnosis of CKD, individual measures of exposure to multiple pollutants, and more control of confounding.