Vitamin B12 is common and unfortunately one cannot rely on serum vitamin B12 to detect a deficiency. Vitamin B12 is carried in the blood by either of 2 proteins: haptocorrin and holotranscobalamin. While the majority of vitamin B12 is carried by haptocorrin, this vitamin B12 is considered inactive* . A serum vitamin B12 test cannot differentiate between the active and inactive form and as a result while the level may appear healthy, the active form of vitamin B12 may be significantly low.
WHICH TEST IS BEST TO IDENTIFY VITAMIN B12 DEFICIENCY?
The most direct way to detect vitamin B12 deficiency is to measure your active form of B12: holotranscobalamin. Biolab in UK offers that test.
If that test is not available to you, your 2nd best option is to measure your homocysteine levels. Homocysteine is a protein humans synthesize in their body and it’s considered one of the most significant biomarkers of cardiovascular health. Its production relies on the availability of vitamin B12, folate & protein.
As multiple other factors though affect the levels of Homocysteine, one cannot drive conclusive results for her vitamin B12 just knowing her homocysteine level.
Which symptoms indicate vitamin B12 deficiency?
Vitamin B12 plays a critical role in the methylation cycle  (which consists of the folate & methionine cycle). As a result, any problems associated with methylation may be driven due to:
- low vitamin B12 intake (important for vegans and vegetarians)
- poor absorption (relevant for those with poor gastrointestinal function)  or
- compromised metabolism (possibly due to MTR & MTRR polymorphisms)
* due to the fact that haptocorrin receptors are found mainly in the liver.
- Morkbak, A.L., Poulsen, S.S. and Nexo, E., 2007. Haptocorrin in humans. Clinical Chemical Laboratory Medicine, 45(12), pp.1751-1759.
- Schjønsby, H., 1989. Vitamin B12 absorption and malabsorption. Gut, 30(12), p.1686.
- Miller, A., Korem, M., Almog, R. and Galboiz, Y., 2005. Vitamin B12, demyelination, remyelination and repair in multiple sclerosis. Journal of the neurological sciences, 233(1), pp.93-97.
- Refsum, H., Nurk, E., Smith, A.D., Ueland, P.M., Gjesdal, C.G., Bjelland, I., Tverdal, A., Tell, G.S., Nygård, O. and Vollset, S.E., 2006. The Hordaland Homocysteine Study: a community-based study of homocysteine, its determinants, and associations with disease. The Journal of nutrition, 136(6), pp.1731S-1740S.