Clinical and Interpretive

Measurement of B12 is useful in the investigation of macrocytic anemia and in the 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.

A serum vitamin B12 level <180 ng/L may cause megaloblastic anemia and/or peripheral neuropathies. Vitamin B12 level <145 ng/L is considered evidence of vitamin B12 deficiency. Follow-up with tests for antibodies to intrinsic factor (IFBA / Intrinsic Factor Blocking Antibody, Serum) are recommended to identify this potential cause of vitamin B12 malabsorption. For specimens without antibodies, follow-up testing of vitamin B12 tissue deficiency by measuring MMA (MMAS / Methylmalonic Acid [MMA], Quantitative, Serum) and/or homocysteine (HCYSP / Homocysteine, Total, Plasma) may be indicated if the patient is symptomatic.

Patients with serum B12 levels between 145 and 180 ng/L are considered borderline and should be evaluated further by functional tests for vitamin B12 deficiency. The plasma homocysteine level is a good screening test. A normal level effectively excludes vitamin B12 and folate deficiency in an asymptomatic patient. However, the test is not specific and many situations can cause an increased level. In contrast, an increased serum MMA level is more specific for cellular-level B12 deficiency and is not increased by folate deficiency.

Ordering ACASM / Pernicious Anemia Cascade simplifies the evaluation of B12 deficiency and will ensure that additional testing is performed in patients with a decreased vitamin B12 level:

The cascade begins with serum B12 measurement. If the vitamin B12 level is <150 ng/L, intrinsic factor-blocking antibody (IFBA) testing is automatically performed. If the IFBA test is negative or indeterminate, gastrin level is evaluated. If the serum vitamin B12 level is 150 to 400 ng/L, methylmalonic acid is measured. If the methylmalonic acid is >0.40 units/L, IFBA testing is performed. If the serum vitamin B12 level is >400 ng/L, no further testing is performed.

Patients taking vitamin B12 supplementation may have misleading results. Recent vitamin B12 administration could result in normal or elevated serum concentrations. Many other conditions are known to cause an increase or decrease in the serum vitamin B12 concentration including:

Increased serum B12
-ingestion of vitamin C
-ingestion of estrogens
-ingestion of vitamin A
-hepatocellular injury
-myeloproliferative disorder
-uremia

Decreased serum B12
-pregnancy
-aspirin
-anticonvulsants
-colchicine
-ethanol ingestion
-contraceptive hormones
-smoking
-hemodialysis
-multiple myeloma

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