We observed that the 5-formyltetrahydrofolate that was not recovered as pABG was converted to 10-formylfolic acid.
A key criterion for measuring serum folate as pABG equivalents is the absence of pABG in native serum or the presence of only trace amounts.
We also screened for pABG in 48 serum samples with increased serum creatinine concentrations (130957 [micro]mol/L).
Our observation of the absence of pABG in native serum is in disagreement with a recent report that stated amean pABG concentration in human serum of about 10 nmol/L (31).
This observation demonstrates the essentially complete conversion of 5-methyltetrahydrofolate and no further degradation of pABG to pABA under the conditions we established for oxidation and acid hydrolysis.
apABG did not interfere with the assay, whereas about 40% of the methotrexate was recovered as pABG (data not shown).
We compared folate concentrations obtained with the pABG assay in serum and in EDTA-treated plasma (n = 48).
Measurements of folate concentrations in 399 fresh serum samples from healthy blood donors obtained with the pABG method and with a microbiologic assay showed good agreement (Fig.
We used serum samples (n = 83) collected between 1973 and 1980 that we had retrieved from the JANUS biobank (25) to investigate whether the pABG assay measured folate in stored serum samples containing low folate concentrations due to folate degradation.
In summary, serum folate can be measured with a method that is based on the quantitative conversion of folate to pABG by oxidation and mild acid hydrolysis, whereas a method that involves strong acid hydrolysis of serum folate to pABA was not feasible because of the presence in serum of pABA precursors not related to folate.
Simple method for the quantitative analysis of endogenous folate catabolites p-aminobenzoylglutamate (pABG) and its acetamido (apABG) derivative in human serum and urine by liquid chromatography-tandem mass spectrometry.
 Nonstandard abbreviations: pABA, p-aminobenzoic acid; LC-MS/MS, liquid chromatography-tandem mass spectrometry; pABG, p-aminobenzoylglutamate; apABG, p-acetamidobenzoylglutamate.