[Evaluation of the enzymatic method using oxalate oxidase for urinary oxalate assay]

The principle of this kit method is that urinary oxalate is extracted and subsequently assayed by measuring the amount of hydrogen peroxide produced in an oxidation reaction catalyzed by oxalate oxidase. The reproducibility and accuracy of the method were tested: the within-run and day-to-day coefficients of variation were 5.4-20.0% and 16.1-18.0%, respectively, and the overall recovery rate of the added oxalate (5-25 mg/l) was 40-50%. These abnormally low recovery rates may be related to the presence of sulfate and phosphate in the extracted fluid. Therefore, the above method was modified by performing a recovery test by adding 25 mg/l oxalate to all urine samples. By the modified method, the correlation coefficient obtained between this method and the ion-chromatographic method was 0.851 (p less than 0.01). Urinary pretreatment with either acid ferric chloride or EDTA yields a higher recovery than with HCl. However, a good correlation of oxalate values is consistently observed for HCl-processed urine as measured by the above two methods. If the interference of ascorbic acid is negligible, no special urinary treatment except for HCl is necessary. The 24-hour urinary oxalate excretions were 24.4 +/- 9.1 mg (mean +/- SD) in 8 healthy males and 19.9 +/- 10.3 mg in 24 calcium-stone formers (21 males and 3 females).