The determination of plasma oxalate concentrations using an enzyme/bioluminescent assay

An enzyme/bioluminescent assay for the determination of oxalate in plasma is described in which NADH, a reaction product of the enzymic degradation of oxalate by oxalate decarboxylase and formate dehydrogenase, is determined using a commercially available bioluminescent system. In contrast to most previously documented methods, this sensitive and specific assay requires minimal sample preparation allowing oxalate concentrations to be determined within 2 h of sample collection. The limit of detection for plasma samples is 0.8 mumol/l. The recovery of oxalate added to plasma averaged 99%. The inter-batch coefficient of variation, calculated by analysis of a plasma sample from a uraemic patient (oxalate concentration = 45.8 mumol/l) on 8 occasions, over a period of 5 wk, was 3.2%. Plasma oxalate concentrations in 35 normal subjects ranged from less than 0.8-1.5 mumol/l, which is in excellent agreement with values obtained by in vivo isotope dilution studies. Plasma oxalate was found to be strikingly elevated in a group of uraemic patients maintained on regular haemodialysis.