Optimum concentration of metaphosphoric acid as the deproteinizing reagent for enzymatic analysis of ascorbic acid in serum

Prior to the assay of ascorbic acid (AA) in serum, the specimens are deproteinized with metaphosphoric acid (MPA). We report here the optimum concentration of MPA as the appropriate deproteinizing reagent for the enzymatic analysis of AA in serum. We used an ascorbate oxidase method on an automated analyzer. MPA in final concentrations from 0 to 50 g/L did not interfere with a calibration run having an AA concentration of 25 g/L. The AA concentrations in 29 serum specimens deproteinized with 10 g/L MPA or those obtained without deproteinization averaged 0.8-1.2 mg/L higher than those deproteinized by 20 to 50 g/L MPA. We found a good correlation between 20 and 50 g/L MPA, r= 0.987-0.998, P < 0.001 , but no agreement with 10 g/L MPA or by direct analysis without deproteinization. We recommend the use of 40 g/L MPA in the final reaction mixture for enzymatic analysis of AA in serum.