A semi-automatic enzymic method for estimating urinary oxalate.

A two-enzyme method is presented for estimating urinary oxalate by continuous-flow analysis. Oxalate is decarboxylated, yielding formate, which is subsequently oxidized in the presence of NAD+, yielding NADH. The NADH, measured colorimetrically, is proportional to the amount of oxalate originally present in the sample. Urine is pretreated by precipitation of the oxalate and extraction of the precipitate in citrate buffer. A correction factor for the extraction efficiency is given by the recovery of [U-14C]oxalic acid added to aliquots of the 24-h urine specimen. The normal range obtained by this method was 0.20 to 0.52 mmol/24h (18 to 47 mg/24h) with a mean value of 0.37 mmol/24 h (33 mg/24h). The between-assay CV was 7.6% (n = 12), within-assay CV 3.2% (N = 25). Unlabeled oxalate (0.5 and 1.0 mol/L) added to 28 urine samples gave a mean analytical recovery of 95% (SD 10%).