The real and apparent plasma oxalate.

A method for the determination of human plasma oxalate concentration by an enzymatic assay procedure is described using deproteinised plasma. The apparent concentration of oxalate in 20 normal subjects was 1.1--16.0 mumol/l (mean 7.78; S.D. 3.96). It was suspected that these results might be too high, due to the possible conversion of glyoxalate to oxalate, and this reaction was clearly demonstrated to occur in whole blood in vitro. Inhibitors (boric acid, DL-beta-phenyllactic acid, and allopurinol) of this oxidation were therefore added to the freshly taken blood samples, prior to assaying by the same technique. The plasma oxalate concentration from normal subjects was then found to be 0--5.4 mumol/l (mean 2.26; S.D. 1.67). It is concluded that normal blood spontaneously generates oxalate on standing, and the higher values obtained by other in vitro methods must be fallacious.