Estimation of Urinary Oxalate by the Method of Isotope Dilution

Abstract

DURING the course of a series of metabolic experiments in which 1-¹³C-ascorbic acid and 1-¹³C-glycine were fed to a normal adult¹ and hyperoxaluric patients² daily samples of urinary oxalic acid were isolated as calcium oxalate for estimation of the ¹³C enrichment. It was found expedient to determine the daily levels of excretion by the method of isotope dilution, using the specific ¹⁴C-activity of the oxalate samples prior to combustion to CO₂ for mass spectrometric analysis. The amounts of ¹⁴C and ¹²C added were low and hence did not interfere with the measurement of ¹³CO₂. Previously reported methods for calcium oxalate estimation have the disadvantage that they depend on the complete recovery of oxalate from a sample of the urine3–7. Using the method of isotope dilution, however, the only exacting requirement is the purity of the final product, which is checked by repeated reprecipitation to constant specific activity.