[C13/C12 Ratios in human urinary calculi]

Oxalate, uric acid, and phosphate stones have been analyzed for their carbon isotope composition. The oxalate stones show delta13C values between -17.0 and -19.5%, the uric acid stones between -14.9 and -19.4%, and the phosphate stones between -13.0 and -23.9%. It is proposed that endogenic rather than exogenic sources are responsible for the 13C/12C ratios of the stones. The isotopic composition of the phosphate stones seems to be influenced primarily by bacterial activity.