Relationship Between Glomerular Filtration Rate and 24-Hour Urine Composition in Patients With Nephrolithiasis

Objective To examine the relationship between GFR and 24-hour urine composition in patients with nephrolithiasis to understand how renal function may affect stone risk. Alterations in glomerular filtration rate (GFR) are associated with a number of physiological changes. Methods A retrospective, institutional review board–approved review of patients from 2 metabolic stone clinics was performed. One-way analysis of variance and multivariate linear regression models were used to evaluate the relationship between GFR quintile and 24-hour urine composition. Results A total of 403 patients (241 male, 162 female) with a mean age of 52.6 ± 14.2 years were included in the study. On univariate analysis, decreasing GFR by quintile was associated with significant reductions in urine calcium, citrate, supersaturation of calcium oxalate, and supersaturation of calcium phosphate ( P < .05 for each). In multivariate linear regression models, decreasing GFR by quintile was associated with significant decreases in urine calcium (β = −11.2, 95% CI = −18.3 to 4.01), urine citrate (β = −32.4, 95% CI = −54.1 to 10.8), oxalate (β = −1.83, 95% CI = −2.85 to 0.81), supersaturation of calcium oxalate (β = −0.58, 95% CI = 0.84 to 0.33) and supersaturation of calcium phosphate (β = −0.09, 95% CI = 0.17 to 0.02), as well as an increase in urine magnesium (β = 3.40, 95% CI = 0.7 to 6.1). Conclusion Reduction in GFR is associated with decreased urine calcium, oxalate, and citrate, and increased urine magnesium. These findings have implications for treatment of patients with stone disease and impaired renal function.