Supplementary Material for: Risk Factors for Common Kidney Stones Are Correlated with Kidney Function Independent of Stone Composition

Background: Kidney stone type varies with age, sex, season, and medical conditions. Lower estimate glomerular filtration rate (eGFR) leads to changes in urine chemistry, and risk factors for kidney stones are thought to vary by stone type. We explore the association between eGFR, urine risk factors, and common stone compositions. Methods: This was a retrospective cohort study of 811 kidney stone patients seen at Yale Medicine between 1994-2021 with serum chemistries and 24-hour urine chemistries matched within one year of baseline stone analysis. Patients’ eGFR was calculated using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) 2021 equation. Demographics and medical history were compared by Chi-squared tests. 24-hour urine chemistries and stone analyses were analyzed by one-way ANOVA. Linear regressions were performed to control for demographics, comorbidities, and stone composition. Results: With lower eGFR, the proportion of calcium stones declined while uric acid stones increased. On univariable analysis, lower eGFR was associated with lower urine pH, calcium, citrate, uric acid, magnesium, phosphorus, and ammonium. On multivariable analysis, controlling for age, sex, ethnicity, BMI, comorbidities, and stone type, these factors remained significant. Stone formers with lower eGFR had elevated supersaturation for uric acid, but reduced supersaturations for calcium-containing stones. Though urine oxalate was significant on univariable analysis, it was not on multivariable analysis. Discussion/Conclusion: Changes in urine parameters are strongly correlated with eGFR regardless of stone type. Renal function may play a key role in modulating kidney stone risk factors. Strategies to mitigate stone risk may need to vary with kidney function, especially when patient urine or stone composition data is unavailable.