What does the Vitamin B12 (Cobalamin), Serum (Total) test measure?

Why is Vitamin B12 (Cobalamin), Serum (Total) measured?

Serum B12 is the standard first-line screening marker for cobalamin deficiency, but its well-documented limitations in the 200–400 pg/mL range make it a starting point for B12 status assessment rather than a definitive diagnostic marker. B12 deficiency produces two distinct clinical syndromes that may occur independently: haematological megaloblastic anaemia from impaired thymidylate synthesis in rapidly dividing erythroid and gastrointestinal precursor cells (producing macrocytosis, hypersegmented neutrophils, megaloblastic bone marrow changes, and pancytopenia in severe cases); and neurological subacute combined degeneration from myelin synthesis impairment in the posterior and lateral spinal cord columns (producing progressive paraesthesia, proprioceptive loss, ataxia, and cognitive impairment that may be irreversible if untreated). A critical clinical principle is that neurological B12 deficiency may occur without haematological abnormality — folate co-supplementation masks the haematological expression while neurological deterioration continues — making B12 assessment mandatory before folate supplementation is initiated in any patient with macrocytic anaemia or neurological symptoms. Serum B12 is the primary marker ordered when B12 deficiency is suspected and guides decisions about supplementation initiation; methylmalonic acid (MMA) and homocysteine are the functional confirmation markers for borderline serum B12 values.

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 200 pg/mL is widely recognised in the clinical literature as insufficiently sensitive for identifying functional B12 deficiency; many laboratories use 200 pg/mL as the lower limit, but patients with serum B12 between 200 and 400 pg/mL may have functional deficiency detectable by elevated methylmalonic acid (MMA) and/or total homocysteine. Confirm whether Access Medical Laboratories designates a 'borderline' or 'grey zone' range (typically 200–400 pg/mL) on their report to prompt functional marker follow-up, and confirm assay methodology — competitive binding luminescence immunoassay (CBLIA) is the dominant platform.

Functional medicine optimal

400–900 pg/mL (functional medicine and preventive neurology optimal — the upper half of the reference range reflects replete tissue stores with adequate methylation and neurological protection capacity; many functional medicine practitioners target above 600 pg/mL for neurological support and above 800 pg/mL during active neurological recovery from B12 deficiency). # REVIEWER FLAG: The 400–900 pg/mL functional optimal is used in integrative and preventive medicine but is not a standardised clinical guideline threshold — mainstream guidelines define deficiency at <200 pg/mL with borderline concern at 200–400 pg/mL. Reviewer to confirm appropriate framing for this platform's practitioner audience.

Units

pg/mL, pmol/L

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