Effect of Socioeconomic Status on 24-Hour Urine Composition in Patients With Nephrolithiasis

Abstract

To examine the relationship between the poverty and education levels and 24-hour urine composition in patients with nephrolithiasis because little is known about the relationship between socioeconomic status and kidney stone risk. A retrospective review was performed of patients evaluated at 2 metabolic stone clinics. The poverty level (ie, percentage of those living below the poverty level) and education level (ie, percentage of those with a high school education or greater) for each postal code were determined from the U.S. Census Bureau data. Multivariate linear regression analysis was used to examine the relationship between the poverty and education levels and 24-hour urine composition. A total of 435 patients were included in the present study. Of the 435 patients, 173 were women and 262 were men (40% women), the mean age was 52.5 ± 14.4 years, and the mean body mass index was 28.6 ± 6.5 kg/m(2). The mean percentage of those below the poverty level was 8.2% ± 6.2%, and the mean percentage of those with a high school education or greater was 87.4% ± 7.4%. On multivariate linear regression analysis, an increasing local poverty level was associated with significant increases in urine calcium (β = 1.51, 95% confidence interval [CI] 0.16-2.86). A decreasing local level of education (ie, decreasing percentage of those with a high school diploma or greater) was associated with significant increases in urine calcium (β = 1.26, 95% CI 0.10-2.42), supersaturation of calcium oxalate (β = 0.04, 95% CI 0.006-0.09), and supersaturation of calcium phosphate (β = 0.013, 95% CI 0.0002-0.03). No other associations were found between the poverty and education levels and any urine constituents or supersaturations. In the present study of patients with stone formation, increasing poverty was associated with increased urine calcium, and increasing education appeared to be protective by decreasing urine calcium and the supersaturation of calcium oxalate and calcium phosphate. Additional studies are important to elucidate the mechanisms underlying these findings.