Test ID: B12    
Vitamin B12 Assay, Serum

Investigation of macrocytic anemia

Workup of deficiencies seen in megaloblastic anemias

Vitamin B12 (cobalamin) is necessary for hematopoiesis and normal neuronal function. In humans, it is obtained only from animal proteins and requires intrinsic factor (IF) for absorption. The body uses its vitamin B12 stores very economically, reabsorbing vitamin B12 from the ileum and returning it to the liver; very little is excreted.

Vitamin B12 deficiency may be due to lack of IF secretion by gastric mucosa (eg, gastrectomy, gastric atrophy) or intestinal malabsorption (eg, ileal resection, small intestinal diseases).

Vitamin B12 deficiency frequently causes macrocytic anemia, glossitis, peripheral neuropathy, weakness, hyperreflexia, ataxia, loss of proprioception, poor coordination, and affective behavioral changes. These manifestations may occur in any combination; many patients have the neurologic defects without macrocytic anemia.

Pernicious anemia is a macrocytic anemia caused by vitamin B12 deficiency that is due to a lack of IF secretion by gastric mucosa.

Serum methylmalonic acid and homocysteine levels are also elevated in vitamin B12 deficiency states.

180-914 ng/L

Concentration of vitamin B12 <180 ng/L may cause megaloblastic anemia and/or peripheral neuropathies.

Vitamin B12 concentrations <150 ng/L are considered evidence of vitamin B12 deficiency.

Vitamin B12 concentrations between 150 to 300 ng/L are considered borderline.

Follow-up testing for antibodies to intrinsic factor (IF) (IFBA/9335 Intrinsic Factor Blocking Antibody, Serum) is recommended to identify this potential cause of vitamin B12 malabsorption.

For specimens without antibodies, follow-up testing of vitamin B12 tissue deficiency by measuring methylmalonic acid (MMA) (MMAS/80289 Methylmalonic Acid [MMA], Quantitative, Serum) and/or homocysteine (HCYSP/80379 Homocysteine, Total, Plasma) may be indicated if the patient is symptomatic.

A normal serum concentration of vitamin B12 does not rule out tissue deficiency of vitamin B12. The most sensitive test for vitamin B12 deficiency at the cellular level is the assay for MMA. If clinical symptoms suggest deficiency, measurement of MMA and homocysteine should be considered, even if serum vitamin B12 concentrations are normal.

See Pernicious Anemia Testing Cascade in Special Instructions.

Patients taking vitamin B12 supplementation may have misleading results.

Many other conditions are known to cause an increase or decrease in the serum vitamin B12 concentration including:

The evaluation of macrocytic anemia requires measurement of both vitamin B12 and folate levels; ideally they should be measured simultaneously.

Some patients who have been exposed to animal antigens, either in the environment or as part of treatment or imaging procedure, may have circulating antianimal antibodies present. These antibodies may interfere with the assay reagents to produce unreliable results.

1. Babior BM: The megaloblastic anemias. In Hematology. Fifth edition. Edited by WJ Williams, E Beutler, MA Lichtman, et al. New York, McGraw-Hill Book Company, 1995, pp 471-490

2. Shenkin A, Baines M, Fell GS, et al: In Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Edited by CA Burtis, ER Ashwood, DE Bruns. St. Louis, Elsevier, Inc., 2006, pp 1100-1105

3. Klee GG: Cobalamin and folate evaluation: measurement of methylmalonic acid and homocysteine vs vitamin B12 and folate. Clin Chem 2000 August;46(8 Pt 2):1277-1283

• Pernicious Anemia Testing Cascade