Vitamin B12 Deficiency - Diagnosis

Diagnosis

Serum B₁₂ levels are often low in B₁₂ deficiency, but if other features of B₁₂ deficiency are present with normal B₁₂ then further investigation is warranted. One possible explanation for normal B₁₂ levels in B₁₂ deficiency is antibody interference in people with high titres of intrinsic factor antibody. Some researchers propose that the current standard norms of vitamin B₁₂ levels are too low. In Japan, the lowest acceptable level for vitamin B₁₂ in blood has been raised from about 200 pg/mL (145 pM) to 550 pg/mL (400 pM).

Serum vitamin B₁₂ tests results are in pg/mL (picograms/millilitre) or pmol/L (picomoles/litre). The laboratory reference ranges for these units are similar, since the molecular weight of B₁₂ is approximately 1000, the difference between mL and L. Thus: 550 pg/mL = 400 pmol/L.

Serum homocysteine and methylmalonic acid levels are considered more reliable indicators of B₁₂ deficiency than the concentration of B₁₂ in blood. The levels of these substances are high in B₁₂ deficiency and can be helpful if the diagnosis is unclear. Approximately 10% of patients with vitamin B₁₂ levels between 200–400pg/l will have a vitamin B₁₂ deficiency on the basis of elevated levels of homocysteine and methylmalonic acid.

Routine monitoring of methylmalonic acid levels in urine is an option for people who may not be getting enough dietary B₁₂, as a rise in methylmalonic acid levels may be an early indication of deficiency.

If nervous system damage is suspected, B₁₂ analysis in cerebrospinal fluid is possible, though such an invasive test should be considered only if blood testing is inconclusive.

The Schilling test has been largely supplanted by tests for antiparietal cell and intrinsic factor antibodies.