Holotranscobalamin (holo-TC), also known as active B12, is the only form of vitamin B12 that is taken up and used by the cells in the body. It accounts for approximately 10% of the circulating vitamin B12 and is the earliest marker showing vitamin B12 depletion [1,2].
Vitamin B12 deficiency is generally suspected based on related symptoms, clinical findings, and laboratory results and is confirmed by measuring vitamin B12 levels. However, current vitamin B12 measurement methods may miss the lack of vitamin B12 in some cases. These methods, based on competitive binding luminescence assays, have been used since 1990. The assay uses binding to intrinsic factor (IF) following dissociation from the binding proteins, with a readout based on the remaining amount of unbound IF. The main problem with these assays is caused by the presence of IF antibodies in the test sample. IF antibodies may bind the test IF reagent and if there is a failure in the denaturation step intended to denature IF-blocking antibodies, spuriously normal or increased vitamin B12 levels can be measured [3,4]. Low vitamin B12 levels can be measured as false normal or false high, especially in pernicious anemia, due to excessive amounts of anti-intrinsic factor antibodies present in the serum [5,6,7].
In the light of data from the available literature, a normal or high vitamin B12 measurement does not exclude vitamin B12 deficiency in cases when vitamin B12 deficiency is suspected. In such an instance, holo-TC and/or metabolic tests, such as homocysteine or methylmalonic acid, may be considered for patients for whom there is a high suspicion of pernicious anemia in the absence of a low vitamin B12 level. Additionally, an alternate approach involves providing vitamin B12 treatment and confirming or eliminating vitamin B12 deficiency according to the response status.