Serum folate

Synonym: s-Folate.
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.
What is measured: Biologically active folate species in serum/plasma. The prevailing folate form in serum/plasma is 5-methyltetrahydrofolate.

What is measured on the same platform, click here.
Platform F: s-Folate, e-Folate, s-B12

Performance of the assay

Lower limit of detection (LOD): 2 nmol/L.
Within-day CV: 4 %; between-day CV: 5 %.


Assessment of folate status (2).

Specimen, collection and processing

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.


Folate in serum/plasma is degraded at room temperature, but also in samples frozen at -20 °C. At room temperature and in the absence of ascorbic acid, folate is substantially degraded within days, a process that proceeds faster in EDTA plasma than in serum. In samples stored at -20 °C, there seems to be a decline in concentration (to about 70 %) during the first year, and the concentration is relatively stable thereafter. In some samples stored frozen for years, folate is not detected.
Since mTHF is the prevailing folate species in serum/plasma, the kinetics of the degradation follow that of mTHF.

Transportation; for general instruction on transportation, click here.

Frozen, on dry ice.

Reported values, interpretation

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.