Interference-free sample preparation for the determination of plasma oxalate analyzed by HPLC-ER: preliminary results from calcium oxalate stone-formers and non-stone-formers

Abstract

Background: Oxalate generation at pH-values above 5.0 and an oxalate–protein binding in acidified plasma would appear to complicate the determination of oxalate in plasma. Methods: To avoid complex sample preparation we used a high-performance liquid chromatographic system with an inline enzyme reactor (HPLC-ER) containing immobilised oxalate oxidase. The detection limit was 0.68 μmol/l. Blood was drawn in lithium–heparin vessels and immediately centrifuged at 4 °C. The yielded plasma was ultrafiltered using a Centrisart-I-tube. To inhibit oxalate generation by ascorbic acid, the ultrafiltrate was acidified with 1 mol/l hydrochloric acid during ultrafiltration at 4 °C. The liquid thus yielded was used for HPLC-ER analysis. Blood samples were obtained from 133 healthy adults (63 men, 70 women, aged 20–94 years) with no history of renal disorder and from 79 patients (53 men, 26 women, aged 19–77 years) with a history of calcium oxalate stone formation. Results: Mean plasma oxalate was 2.65±2.31 μmol/l for healthy subjects and 4.21±0.56 μmol/l for stone formers. Conclusions: Analysis yielded no significant differences between males and females. A correlation between age and plasma oxalate was found for the healthy adults ( p<0.001).