Biologically active folate species in serum/plasma. The prevailing folate form in serum/plasma is 5-methyltetrahydrofolate.
Method(s): Microbiological assay, using a chloramphenicol-resistant strain of Lactobacillus casei (1). The assay has been adapted to a microtiter plate format and is carried out by a robotic workstation. Notably, samples containing antibiotic(s) that inhibit(s) the growth of Lactobacillus casei may cause assay interference.
Patient/subject: Serum folate increases after at folate rich meal, and the concentration reflects (in contrast to erythrocyte folate) short-term folate status. Some antibiotics may inhibit growth of Lactobacillus casei, and may cause artificial, low folate levels.
Matrix: Serum is preferred. Folate is one of few biomarkers that degrade faster in EDTA plasma than in serum.
Volume: Minimum volume is 100 µL, but 250 µL is optimal and allows reanalysis.
Preparation: The serum concentration of mTHF increases markedly in hemolytic samples, which is explained by release of high amounts of cellular folate into the serum/plasma fraction. Optimal procedure for sample handling involves addition of ascorbic acid and rapid freezing at -80 °C.
Reported values: > 7.5 nmol/L.
Intraclass correlation coefficient (ICC): 0.56.
1. Molloy, A.M., and Scott, J.M. (1997). Microbiological assay for serum, plasma, and red cell folate using cryopreserved, microtiter plate method. Methods Enzymol 281, 43-53.
2. Bailey, L.B., Stover, P.J., McNulty, H., Fenech, M.F., Gregory, J.F., Mills, J.L., Pfeiffer, C.M., Fazili, Z., Zhang, M., Ueland, P.M., et al. (2015). Biomarkers of nutrition for development-folate review. J Nutr 145, 1636S-680S.