[A modification of the estimation of urinary oxalate using a Sigma kit]

The Sigma kit for estimating urinary oxalate is an enzymatic procedure. However, some errors were encountered using the standard assay system of the kit. Firstly, an overestimate of oxalate may arise from the oxidation of ascorbate during the alkaline wash stage of the extraction of oxalate from urine. Secondly, an underestimate of oxalate may occur because of incomplete extraction of oxalate. A modified assay system for measurement of urinary oxalate using the kit is reported. The following points were modified: urine was diluted two-fold with 0.2 M EDTA and and 0.2 M citrate buffer (pH 3.0), oxalate from urine was extracted with 0.06 N sodium hydroxide to prevent the overestimation by the oxidation of ascorbate, and a plate mixer and addition of a small magnet to the vial were used in the steps of both absorption and extraction of oxalate to keep the accuracy of the estimation. The linearity of standard curve, reproducibility and recovery rates of the modified method were studied, and good results were obtained (linearity; r = 0.999, CV of reproducibility; 5.3%, recovery rate; 101% (300 microM) and 103% (600 microM). A good correlation was seen between the modified Sigma method and high performance liquid chromatography (r = 0.991).