The determination of urinary oxalate by gas chromatography

A new quantitative determination method for urinary oxalate by gas chromatography has been established. This method consists of concentration of urine to dryness followed by methylation of the oxalic acid with 7 per cent hydrochloric acid-methanol, subsequent extraction with chloroform and gas chromatographic analysis. The advantages of the technique are smaller sample sizes (only 5 ml.), rapidity (within 2 to 3 hours) and accuracy (99.14 plus or minus 1.34 per cent recovery). In addition, the treatment of the sample is simple and no specialized equipment is required. Therefore, this method is recommended to clinicians for routine determination of oxalic acid in urine. Urinary oxalate content was analyzed with this method in 62 male patients with calcium oxalate stones. Urinary oxalate concentration and excretion in 33 patients with recurrent stones were significantly higher than in the 22 controls using Welch's test.