Renal Metabolic Defects in Patients with Recurrent Unilateral Nephro- and Ureterolithiasis: Insights from Individual Renal Unit Sampling

Background and Objective: Urolithiasis is a highly prevalent condition influenced by environmental, dietary, and genetic factors. Recurrent unilateral stone formation raises the question of whether localized renal metabolic abnormalities contribute to lithogenesis. Conventional 24-hour urine analyses provide a global assessment of urinary solutes and may overlook kidney-specific differences. The objective of this study was to determine whether distinct urinary chemistry differences exist between stone-bearing and contralateral kidneys in patients with recurrent unilateral urolithiasis. Methods: This monocentric prospective study was conducted at the Department of Urology, Lausanne University Hospital (Switzerland). Adult patients undergoing elective ureteroscopy (URS) for recurrent unilateral urolithiasis were enrolled. Urine was collected separately from each kidney by selective ureteral catheterization, and samples were analyzed for urinary lithogenic parameters. Paired comparisons were performed using repeated measures ANOVA. Key Findings and Limitations: Fifteen patients were included (median age 50 yr; 67% male; median BMI 24.2 kg/m²), each with a median of three prior stone episodes. Surgical approaches included rigid (47%), flexible (27%), and combined (27%) URS, with a median operative time of 40 min. Calcium oxalate was the predominant stone type (60%), followed by mixed calcium oxalate–phosphate (27%) and uric acid stones (13%). No significant differences were found in urinary chemistry between stone-bearing and contralateral kidneys (all p > 0.10). Subgroup analyses by stone type suggested trends toward lower potassium excretion in mixed stones and reduced chloride in urate stones, though these were not statistically significant due to small subgroup sizes. Limitations include the small sample size and single-center design. Conclusions and Clinical Implications: No significant unilateral metabolic asymmetries were detected in urinary solute handling among patients with recurrent unilateral urolithiasis. These findings support the continued use of standard 24-hour urine testing as an adequate metabolic evaluation tool in most clinical scenarios. Kidney-specific urine sampling may be considered only in selected cases with persistent ipsilateral recurrence despite normal systemic evaluation, or within research protocols exploring pathophysiological mechanisms of stone formation.