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.