What does the Vitamin B12 (Cobalamin) test measure?

Why is Vitamin B12 (Cobalamin) measured?

Serum B12 reflects total circulating cobalamin and is the standard first-line marker for deficiency assessment, which manifests as megaloblastic anaemia, peripheral neuropathy, subacute combined degeneration, and cognitive change. Total B12 has recognised limitations in the indeterminate zone (200–400 pg/mL), where functional markers — methylmalonic acid and homocysteine — provide clarification. Practitioners use it to investigate macrocytosis, malabsorption, and neurological presentations.

Reference ranges

Standard reference

200–900 pg/mL (148–664 pmol/L) # REVIEWER FLAG: Confirm Quest assay-specific range and indeterminate-zone definition.

Functional medicine optimal

Functional medicine optimal commonly cited as >500 pg/mL (>369 pmol/L) # REVIEWER FLAG: Functional optimal threshold for B12 is not standardised across guidelines — reviewer to confirm.

Units

pg/mL, pmol/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.