The Determination of Oxalate in Urine and Plasma by High Performance Liquid Chromatography

We describe a simple, sensitive assay for oxalate in urine or plasma. Acidified urine is pretreated by dilution with neutral phosphate buffer and passage through a C18 cartridge. Stabilized plasma is diluted with neutral acetate buffer and oxalate extracted using a strong anion exchange cartridge. Treated samples are applied to an ion-paired chromatographic system and oxalate detected electrochemically. Recovery of oxalate from augmented samples exceeded 97% from both urine and plasma. Within- and between-assay coefficients of variation assessed at three concentrations were, respectively, better than 4.1% and 8.4% for urine and 3.9% and 5.2% for plasma. The reference range for urinary oxalate excretion is 109-497 mumols/24 h. The range for plasma oxalate concentration is 0.6-2.8 mumols/L or 0.7-3.9 mumols/L after an overnight fast or without dietary restriction, respectively. Urine and plasma oxalate concentrations from this method, gave correlation coefficients (r) of 0.97 and 0.98, respectively, when compared with those from established oxalate oxidase based assays.