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.

背景：肾结石的类型随年龄、性别、季节和医疗状况而变化。较低的估计肾小球滤过率（eGFR）会导致尿液化学成分的改变，且肾结石的风险因素被认为因结石类型而异。本研究旨在探讨eGFR、尿液风险因素与常见结石成分之间的关联。

方法：本研究为1994年至2021年间在耶鲁医学中心接受治疗的811例肾结石患者的回顾性队列研究，这些患者的血清化学和24小时尿液化学成分在基线结石分析的一年内进行匹配。患者的eGFR使用慢性肾脏病流行病学合作组织（CKD-EPI）2021方程式进行计算。通过卡方检验比较了人口统计学和病史。24小时尿液化学成分和结石分析通过单因素方差分析进行。通过线性回归控制人口统计学、合并症和结石成分。

结果：随着eGFR的降低，钙结石的比例下降，而尿酸结石的比例上升。在单变量分析中，较低的eGFR与较低的尿液pH值、钙、柠檬酸盐、尿酸、镁、磷和铵的浓度相关。在多变量分析中，控制年龄、性别、种族、BMI、合并症和结石类型后，这些因素仍然具有统计学意义。eGFR较低的结石患者尿酸过饱和度升高，但含钙结石的过饱和度降低。尽管尿液草酸盐在单变量分析中具有统计学意义，但在多变量分析中不具有统计学意义。

讨论/结论：无论结石类型如何，尿液参数的变化与eGFR之间的相关性均非常强。肾功能可能在调节肾结石风险因素中发挥关键作用。降低结石风险的战略可能需要根据肾功能的变化而变化，尤其是在患者尿液或结石成分数据不可用的情况下。