An excerpt from the New England Journal of Medicine
found at :
Explains several problems with measuring serum cobalamin (Vitamin B12).
The first test performed to confirm the diagnosis of vitamin B12 deficiency is generally measurement of the serum vitamin B12 level. Although an extremely low level (<100 pg per milliliter [<73.8 pmol per liter]) is usually associated with clinical deficiency, such low levels are infrequently observed. Both false negative and false positive values are common (occurring in up to 50% of tests) with the use of the laboratory-reported lower limit of the normal range as a cutoff point for deficiency.4,24,26The high rate of false negative and false positive results may be due to the fact that only 20% of the total measured vitamin B12 is on the cellular delivery protein, transcobalamin; the remainder is bound to haptocorrin, a protein of unknown function.27 Most laboratories now perform automated assays of vitamin B12 on platforms used for many other analytes. There is often poor agreement when samples are assayed by different laboratories or with the use of different methods.31-34Because intrinsic factor is used as the assay-binding protein, anti–intrinsic factor antibodies (which are common in pernicious anemia) must be removed chemically from the sample, which has proved to be problematic in the automated assays.33,34 Recent studies show normal values34 or falsely high values33 of vitamin B12 in many patients with pernicious anemia. New assays of holotranscobalamin (to measure the vitamin B12 saturation of transcobalamin) provide a modest improvement in specificity over that provided by assays of total serum vitamin B12, but they have not been clinically validated27-29 and are not yet available commercially in the United States.
Given the limitations of available assays, clinicians should not use a laboratory's reported lower limit of the normal range to rule out the diagnosis of vitamin B12 deficiency in patients with compatible clinical abnormalities. Clinicians should also recognize that vitamin B12 values are frequently low in patients without other metabolic or clinical evidence of vitamin B12 deficiency (i.e., megaloblastic anemia or myelopathy)."
Additional limitations of the assay are reportedin the attached image from the book Henry's Clinical Diagnosis and Management by Laboratory Methods:
The Reference Range for Vitamin B12. Image from MedScape App.