[A new method to assay urinary oxalic acid using high-performance liquid chromatography].

Measurement of oxalic acid in urine has an important clinical significance because approximately 70% of human urinary calculi are composed of calcium oxalate. Different analytical methods of oxalic acid have been reported, but most are unsuitable for clinical use. Thus we developed a new method to assay oxalic acid in urine using high-performance liquid chromatography (HPLC). This acid was extracted from urine with tri-n-butyl phosphate and converted into the fluorescent derivative by esterification with 9- anthryldiazomethane (ADAM). The reaction mixture containing the oxalic acid derivative can be directly chromatographed on HPLC using an ODS reverse phase type column monitored with a fluorophotometric detector. A linear relationship was observed from 1 to 100 micrograms/ml of standard oxalic acid dissolved in saline. Healthy adults excrete 23.8 +/- 9.0 mg (mean +/- SD) of oxalic acid per day. This method should prove valuable for routine measurement of urinary oxalic acid as it is accurate, simple and specific.