Differences between urinary metabolites in individual renal units in patients with unilateral nephrolithiasis

Abstract

Abstract INTRODUCTION: Twenty-four-hour urine testing is performed to identify urine electrolyte abnormalities that are associated with kidney stone formation. This analysis pools urine from the two kidneys even though in many patients, stones affect only one renal unit. Our goal was to determine if there were significant differences in urine electrolytes between the stone-bearing and the stone-free kidney. METHODS: Fourteen adult patients with unilateral nephrolithiasis scheduled for ureteroscopy or percutaneous nephrolithotomy were enrolled. Following Foley catheter drainage of the bladder, a ureteral access sheath (UAS) was passed into the stone-bearing kidney. Urine was collected simultaneously from the UAS (intervention) and from the Foley catheter (control) for 10-15 minutes. Samples were analyzed for urine stone risk factors. Pairwise comparison was made between the two kidneys using both raw concentration values and concentration values corrected for creatinine. The amount of urine collected determined the type of analysis performed. In those patients where at least 6ml of urine was collected, the samples were split into two samples, one for the addition of thymol and the other for the addition of hydrochloric acid (HCl). RESULTS: Among the 14 patients, there were 11 complete thymol samples and 5 complete HCl samples. There were differences at the individual level between the kidneys. In the HCl samples, all patients had unilateral metabolic differences related to stone formation: higher urinary calcium in three, higher urinary oxalate in one and lower urinary citrate in one patient. CONCLUSIONS: In most patients, there were urinary metabolite differences between the renal units, most notably in urinary calcium concentration.