Decreased radioassay values for folate after serum extraction when pteroylglutamic acid standards are used

Radoiassay values for folate in protein-denautured (extracted) sera are compared with values obtained by individual serum blank correction. Lower folate values are reported for sera extracted at pH 1.3, 7.6, 10.5, and 12.4 when pterolyglutamic acid (PGA) calibration standards are used. The results are unchanged when ascorbate added to the sera is increased from 1 to 20 g/liter and when [3H]PGA is substituted for 125I-labeled PGA. However, a higher mean value for extracted serum is reported when N-5-methyltetrahydrofolic acid calibration standards are substituted for PGA. Because both patients' samples and assay calibrators were subjected to the same extraction methods, our results suggest that serum folate behaves differently from PGA during the extractions.