Bilateral urinary calculi with discordant stone composition.

Objectives: To describe a cohort of bilateral stone formers with significantly different compositions between renal units.

Methods: Patients treated for bilateral nephrolithiasis over a 4-year period (2007-2010) were identified. Stones were categorized by dominant (≥50%) mineralogical component. Patients with significant compositional differences between renal units (discordant stone formers) were compared to patients with a similar stone type in each kidney.

Results: Fifteen of the 59 bilateral stone formers (25.4%) were discordant stone formers with significant differences in stone composition between renal units. Forty-four of the 59 patients (74.6%) had the same stone composition on each side. Thirty percent of discordant stones had calcium phosphate as the dominant stone component. Discordant stone formers were younger, had better renal function, and tended to have a larger stone burden (p < 0.05).

Conclusions: A significant minority of bilateral stone formers form a different type of stone in each kidney. Local or micro-environmental etiologies may explain this phenomenon and may also account for failure of preventive therapy in some patients.