The term "vitamin B12" refers to four cobalamins (Cbl), including methyl-Cbl and adenosyl-Cbl, the two enzyme co-factors of methionine synthase and methylmalonyl-CoA mutase, respectively. Vitamin B12 deficiency produces clinical disorders that include mainly megaloblastic anaemia, peripheral and central neurological manifestations. The clinical significance of low blood B12 concentrations in the absence of manifestations of deficiency is a matter of debate. The biochemical diagnosis of the subclinical and clinical deficiency of vitamin B12 has been enriched by several parameters, including serum methylmalonic acid, homocysteine, and holo-transcobalamine, which have been evaluated over the past two decades.