Variability in stone composition and metabolic correlation between kidneys in patients with bilateral nephrolithiasis

Purpose: To evaluate the clinical significance of discordant stone analyses in patients undergoing bilateral ureteroscopy. Materials and methods: A retrospective chart review was performed for all patients undergoing stone extraction with bilateral ureteroscopy at our institution in an aim to identify patients who had bilateral stone analysis and 24-h urine chemistry data available. Stones were then classified based upon the dominant present (> 50%). Twenty-four hour urinalysis results were reviewed and statistical analysis performed comparing discordant and concordant patient populations, assessing significant differences that would potentially influence clinical management. Results: We identified 79 patients (158 renal units) who had bilateral stones removed at the time of ureteroscopy. The majority of stones were classified as calcium oxalate (CaOx) (60.1%) followed by calcium phosphate (CaP) (27.8%), brushite (5.1%), uric acid (UA) (4.4%), and cystine (2.5%). Discrepancies in stone classifications were present 24% of the time. Evaluation of 24-h urinalysis results demonstrated that patients with CaOx:CaP stone discordance compared to CaOx:CaOx concordant stone formers were more likely to have an elevated pH (p = 0.02) and lower uric acid supersaturation (p = 0.01). Conclusions: Discrepancies in stone mineral content are common in patients with bilateral stone disease. A single stone analysis from one side in the setting of bilateral stone disease is insufficient for management of patients with bilateral renal stones, and may lead to mismanagement when this misrepresented information is utilized in addition to 24-h urinalysis results. At least one stone analysis should be performed from both sides during a bilateral stone extraction procedure.