Three binding proteins are involved in the transport of vitamin B12 around the body – Intrinsic Factor (IF), transcobalamin (TC) and haptocorrin (HC). These binding proteins ensure the efficient uptake of the very small amounts of vitamin B12 available from the diet. When TC and HC bind vitamin B12 the resulting complexes are known as holotranscobalamin (HoloTC) and holohaptocorrin (HoloHC) to distinguish them from the proteins carrying no vitamin.

The major fraction in the circulation, HoloHC, represents 70-90% of vitamin B12 in the blood but is biologically inert. HoloTC represents only 10-30% of vitamin B12 in the blood but is the only form of vitamin B12 that can be taken up by cells in the body hence it’s alternative name Active-B12. The TC protein alone transports vitamin B12 from its site of absorption in the ileum to tissues and cells. The vitamin is then internalised as the Active-B12 complex via a specific receptor-mediated uptake. This process delivers vitamin B12 into the cells of the body and provides the vitamin as a co-enzyme for essential cellular functions such as DNA synthesis.

The measurement of Total Serum B12 suffers from some limitations; in particular, most of the measured B12 is bound to biologically inert HC. Several studies have been published which conclude that HoloTC would be a better indicator of vitamin B12 status than Total Serum B12.


FMABT100 Active-B12 ELISA


Part Number: FMABT100

Method: ELISA

Description: 96-well microtire plate, liquid reagents, 6 calibrators, 2 controls




  • Easy-to-use ELISA technology, compatible with automated systems for improved throughput.
  • Reduced false positive and false negative results associated with Total B12 testing
  • Fewer indeterminate results
  • No interference from Intrinsic Factor antibodies
  • No pre-analytical sample denaturation – remove a source of a variability


Intended Use

The Axis-Shield Active-B12 (Holotranscobalamin) assay is an enzyme-immunoassay (EIA) for the quantitative determination of holotranscobalamin (HoloTC) in human serum. HoloTC (vitamin B12 bound to transcobalamin) is used as an aid in the diagnosis and treatment of vitamin B12 deficiency.


Assay Principle

The microtitre wells are coated with a highly specific monoclonal antibody for Active-B12 (Holotranscobalamin). During the first incubation holotranscobalamin in serum specifically binds to the antibody-coated surface. In the second incubation the Conjugate binds to any captured holotranscobalamin. The wells are then washed to remove unbound components. Bound holotranscobalamin is detected by incubation with the Substrate. Addition of Stop Solution terminates the reaction, resulting in a coloured end-product. The concentration of holotranscobalamin in pmol/L is directly related to the colour generated and can be estimated by interpolation from a dose-response curve based on Calibrators.



More information on Active-B12 can be found at



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