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

Background: Oxalate generation at pH-values above 5.0 and an oxalate-protein binding in acidified plasma would appear to complicate (lie 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 mumol/l. Blood was drawn in lithium-heparin vessels and immediately centrifuged at 4 T. 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 T. 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 mumol/l for healthy subjects and 4.21 +/- 0.56 mumol/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). (C) 2002 Elsevier Science B.V. All rights reserved.