What does the Vitamin B12 test measure?

Why is Vitamin B12 measured?

Serum B12 is the initial test for suspected deficiency, which can present insidiously with macrocytic anaemia, glossitis, neuropsychiatric symptoms (paraesthesiae, ataxia, cognitive impairment), or elevated homocysteine and methylmalonic acid when B12-dependent pathways fail. Low B12 may reflect dietary insufficiency (strict veganism without supplementation), malabsorption (pernicious anaemia, atrophic gastritis, post-gastrectomy, ileal disease or resection, pancreatic insufficiency), or drug interference (metformin, proton-pump inhibitors, H2-receptor antagonists). Because a substantial proportion of circulating B12 is bound to haptocorrin and metabolically inert, a low-normal result in the presence of clinical suspicion warrants measurement of methylmalonic acid and homocysteine to confirm tissue deficiency. Conversely, very high B12 can occur with supplementation, liver disease releasing stored cobalamin, or myeloproliferative disorders increasing haptocorrin.

Reference ranges

Standard reference

200–900 pg/mL (148–664 pmol/L) # REVIEWER FLAG: Confirm Access Medical Laboratories reference range for serum vitamin B12 — the lower limit of the reference range (200 pg/mL) is widely recognised as insensitive for B12 deficiency; many patients with levels between 200 and 400 pg/mL have functional B12 deficiency detectable only by elevated methylmalonic acid and homocysteine. Confirm whether Access Medical Laboratories reports a separate 'grey zone' or borderline range.

Functional medicine optimal

400–900 pg/mL (functional medicine optimal — the upper half of the reference range, reflecting replete stores with adequate methylation capacity; many functional medicine practitioners target >600 pg/mL for neurological protection and cognitive support). # REVIEWER FLAG: Not standardised in mainstream haematology — reviewer to confirm framing and whether a functional optimal range is clinically appropriate to document for this platform.

Units

pg/mL, pmol/L

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