Oxalate measurement in the picomol range by ion chromatography: values in fasting plasma and urine of controls and patients with idiopathic calcium urolithiasis

Abstract

Oxalate was measured by ion chromatography in the ultrafiltrate of heparinized plasma from peripheral venous blood, using a membrane with a cut-off molecular weight (Mr). The following criteria were established: sensitivity 0.7 mumol.l-1; intra- and inter-assay coefficients of variation 4% and 12%, respectively; precision of duplicate determinations (expressed as standard deviation) 0.08 mumol.l-1; overall recovery (oxalate added and diluted, respectively) 100.7%. These qualified the method for assessment of plasma oxalate in healthy human controls (males: n = 12) as well as patients with idiopathic renal calcium urolithiasis (males: n = 22; females: n = 16). Renal calcium urolithiasis patients were subclassified into those with normocalciuria and idiopathic hypercalciuria. In male and female controls the mean values (and range) of plasma oxalate were 1.98 (1.4-2.5) and 1.78 (0.7-2.9) mumol.l-1, respectively. In male controls ultrafiltration (membrane cut off Mr 10,000) revealed that 11-16% plasma oxalate was bound to constituents having an apparent Mr above 10,000, and that with use of membranes with smaller pore size, the ultrafilterability of oxalate decreases further. In renal calcium urolithiasis the following values were elicited (mumol.l-1): male normocalciuria 1.78 (0.8-4.0), idiopathic hypercalciuria 1.58 (1.2-2.2); female normocalciuria 1.69 (0.8-3.6), idiopathic hypercalciuria 1.21 (0.8-2.1). The difference from controls is significant in idiopathic hypercalciuria (males and females). In contrast, in fasting urine of renal calcium urolithiasis the oxalate excretion rate (5-45 mumol per 120 min) and oxalate clearance (21-328 ml per min) resemble those in controls, whereas in renal calcium urolithiasis the fractional oxalate clearance (30-357% of creatinine clearance) tended to higher values (p less than 0.01, in male idiopathic hypercalciuria versus controls). It is suggested that 1) ion chromatography allows the reliable assessment of ultrafiltrable plasma oxalate in health and disease states, 2) in renal calcium urolithiasis this technique may help to elucidate oxalate pathophysiology, especially the mode of renal handling of oxalate.