Individual renal unit urine sampling to identify unilateral metabolic defects

Aims: Preventing kidney stone recurrence relies on detecting and modifying urine chemistry abnormalities. The assumption is that an abnormality is due to a global metabolic defect present in both kidneys. However, we hypothesize that clinically significant unilateral defects may exist. We aimed to identify these patients by sampling urine from each renal unit. Materials and methods: Adults undergoing retrograde upper urinary tract surgery were eligible for inclusion. Excluded were patients with a solitary kidney, suspected urothelial malignancy, or urinary tract infection. Following informed consent, all patients proceeded to the operating room. After induction of anesthesia, cystoscopy with ureteral catheterization was performed with urine collected via gravity drainage for 10 minutes. Urine samples with adequate volume were analyzed for chemistry concentrations. A difference greater than the 75^th percentile between matched pairs was considered significant. For urine pH, a difference of 0.5 was considered significant. Results: A total of 47 patients were screened for eligibility with only 13 (28%) electing to enroll in the study (26 renal units). All subjects underwent successful bilateral ureteral catheterization with no adverse events observed or later reported. The mean (± SD) urine volume captured from the right and left renal units was 5.0 ± 7.4 cm³ and 6.6 ± 6.4 cm³, respectively. Urine was only captured from paired renal units in 8 participants (8/13; 62%). Of these 8 participants, 5 (5/8; 63%) had at least 1 unilateral metabolic defect. Conclusion: Unilateral renal unit urine sampling is safe and feasible. However, captured urine volumes are small and variable, but chemical analysis can still be performed. Unilateral defects in renal electrolyte handling are relatively common, but the clinical implications of these differences are still yet to be determined.