Purpose: The determinations of oxalate in urine and plasma are important in the evaluation and treatment of patients with calcium oxalate nephrolithiasis. Although many analytical methods for determining oxalate have been developed, most of them need complicated sample preparation, and are expensive for routine examination. Especially for estimation of plasma oxalate, much more sensitive measurement is required because of the extremely low concentration. A simple and rapid assay for oxalate in urine and plasma by capillary electrophoresis has been described here, and utilized for assessment of renal oxalate clearance. In addition, simultaneous determination of urinary oxalate and citrate was developed.
Methods: A Waters Quanta 4000E system was used with a detection at 185 nm. Separation was obtained on a fused silica capillary, 60 cm long x 75 microns and 100 microns (i.d.) for urine and plasma samples respectively. Urine samples were diluted with 60 mM hydrochloric acid, and ultrafiltrates of plasma were acidified and diluted with 300 mM boric acid and 50 mM phosphoric acid.
Results: The intraassay coefficient variation was 2.7-4.0% for urinary oxalate, and 1.3-3.9% for citrate. The mean recovery ratio of 0.2 mM oxalate and 1.0 mM citrate added to 10 samples were 99.0% (92.6-107.4%) and 98.4% (91.2-103.9%), respectively. In the determination of plasma oxalate, the minimum detectable limit was 0.9 microM, the coefficient variation was 5.8-16.0%, and the recovery rate was 101.5% (87.8-125.6%). The plasma oxalate levels in 8 adult males were 2.39 +/- 1.46 microM (Mean +/- SD). Renal oxalate clearances with one hour method were 72.9 +/- 20.0 ml/min in 6 healthy controls and 83.2 +/- 27.8 ml/min in 8 stone formers. Oxalate/creatinine clearance ratios in each groups were 0.70 +/- 0.16 and 1.11 +/- 0.34 respectively.
Conclusion: The simultaneous determination of urinary oxalate and citrate was satisfactory. Capillary electrophoresis is suited for routine examination of urinary oxalate and citrate with the advantage on simplicity and economy. The assay of plasma oxalate by this method was also acceptably sensitive, specific under a low temperature and an acidification.