Human plasma oxalate concentration re-examined.

In an attempt to reconcile estimates of plasma oxalate concentration by in vitro and in vivo assays, the in vitro radioenzymatic isotope dilution assay (REIDA) for plasma oxalate concentration was modified by initiating precipitation of calcium oxalate from plasma ultrafiltrates within one hour of phlebotomy. Plasma oxalate concentration form normal males was 4.0 mumol/1, consistent with reported values of 3.0 mumol/1 and 2.3 mumol/1 by enzymatic methods, and 2.8 mumol/1 by a gas chromatographic technique, and lower than 10.0 mumol/1 reported previously by chemical/enzymatic methods. This lower estimate of oxalate results, at least in part, from avoiding in vitro conversion of ultrafilterable substances to oxalate, and from the use of 'fire-cleaned' glassware. No difference in plasma oxalate was observed between age-matched, apparently healthy black and white males or with blood collected in the presence and absence of inhibitors of the conversion of glyoxalate to oxalate.