Synonym: Vitamin B12.
Biologically active cobalamin and not cobalamin analogues (which do not bind to intrinsic factor).
Method(s): Microbiological assay, using colistin sulphate-resistant strain of Lactobacillus leichmannii (1). The assay has been adopted 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 leichmannii may cause assay interference.
Patient/subject: Some antibiotics may inhibit growth of of Lactobacillus leichmannii, and may cause artificial, low cobalamin levels.
Matrix: Serum or plasma.
Volume: Minimum volume is 100 µL, but 250 µL is optimal and allows reanalysis.
Preparation and stability: Cobalamin is light sensitive, but is stable for years in samples frozen at -20 °C or below.
Reported values: > 150 pmol/L.
Intraclass correlation coefficient (ICC): 0.82.
1. Kelleher, B.P., and Broin, S.D. (1991). Microbiological assay for vitamin B12 performed in 96-well microtitre plates. J Clin Pathol 44, 592-95.
2. Green, R., Allen, L.H. Bjørke-Monsen, A-L., Brito, A., Gueant, J-L. Miller, J.W., Nexo, E., Stabler, S., Toh, B-H., Ueland, P.M. and Yajnik, C. (2017). Vitamin B12 deficiency. Nat Rev Dis Prim 3: 17040.