Gas-chromatographic estimation of urinary oxalate and its comparison with a colorimetric method

We describe a simple, specific gas-chromatographic method for urinary oxalate. Its specificity was evaluated by precipitating the oxalate as its calcium salt from urine, followed by methylation of the oxalate with boron trifluoride/methanol and subsequent gas-chromatographic separation and quantitation of the dimethyl oxalate. [U-14C]Oxalate and n-decanoic acid are used as internal standards. Analytical recoveries ranged from 93.8 to 97.7% for oxalate-supplemented urine. Twenty replicate analyses of urines containing typical concentrations of oxalate gave CVs of 10.0, 9.1, and 8.3% for low, medium, and high concentrations, respectively. Day-to-day precision (CV) for single analyses repeated on 20 days was 8.6% for the low urinary oxalate concentration. The lower limit of detection of urinary oxalate is 25 mumol/L. Results by our method correlated well (r = 0.95) with those by a colorimetric method (Clin. Chim. Acta 36: 127, 1972) but averaged 68.4% of those obtained colorimetrically (n = 75 samples, p less than 0.001). The expected range for our method is calculated to be 80 to 500 mumol of oxalate per 24-h urine (mean +/- 1 SD: 280 +/- 100).