Urinary oxalate excretion, as determined by isotope dilution and indirect colorimetry

A simple and reliable method for the determination of urinary oxalate excretion is described. Urinary oxalate is precipitated with calcium chloride, and the oxalate content of the precipitate is measured by an indirect colorimetric method developed by Neas and Guyon in 1972 [1]. For single urine samples, a correction is made for the incompleteness of the precipitation of calcium oxalate by isotope dilution. The range of normal values (5% limits) determined in 52 normal subjects was 0.121-0.325 mmol . 24 h-1 . m-2 for a 1-day collection period and 0.145-0.301 mmol . 24 h-1 . m-2 for a 3-day collection period. The within-assay CV of a control urine with a low oxalate concentration was 9% (n = 7) and the between-assay CV for the same control urine was 12% (n = 6). When the values obtained for oxalate excretion were normalized to body surface area, there was no significant difference between males and females; the main source of variation was the intra-individual variation.