Vitamin B12 (Cobalamin) Test
The Vitamin B12 test measures serum cobalamin, the front-line investigation for suspected deficiency presenting with macrocytic anaemia, neuropathy, or cognitive symptoms. Low results reflect dietary insufficiency, malabsorption (pernicious anaemia, ileal disease, post-surgical states), or drug interference (metformin, acid suppression).
1
Biomarkers
Blood (serum)
Sample
1–2 business days
Turnaround
No
Fasting
What is the B12 test?
The Vitamin B12 Test measures serum cobalamin concentration and is the front-line investigation when B12 deficiency is suspected on clinical or haematological grounds. It is ordered to investigate macrocytic anaemia, unexplained neurological symptoms such as peripheral neuropathy or subacute combined degeneration of the cord, cognitive change in older adults, and glossitis or other mucosal signs of deficiency. Practitioners also order B12 screening in at-risk populations: strict vegans, patients with documented pernicious anaemia or atrophic gastritis, those with a history of gastric or ileal surgery, individuals on long-term metformin or acid suppression, and patients with malabsorption syndromes. Because serum B12 can be low-normal yet functionally insufficient, an equivocal result in the context of compatible symptoms prompts reflex or follow-on testing with methylmalonic acid and homocysteine, both of which rise when cellular B12 is inadequate. The test is not a one-time screen but a tool interpreted in the clinical and haematological context and sometimes serially after treatment is initiated.
What does the B12 test measure?
Cobalamin is an essential water-soluble vitamin with a corrin ring surrounding a central cobalt atom, unique among human vitamins for its requirement of intrinsic factor for ileal absorption. After ingestion, gastric acid and pepsin liberate B12 from food proteins; salivary and gastric R-proteins bind it until pancreatic proteases in the duodenum transfer B12 to intrinsic factor secreted by gastric parietal cells. The intrinsic factor–B12 complex is absorbed in the terminal ileum via the cubilin receptor. In plasma, the majority of cobalamin circulates bound to transcobalamin II (the metabolically active fraction) and haptocorrin. Intracellularly, B12 serves as cofactor for methylmalonyl-CoA mutase in mitochondria and methionine synthase in the cytosol, linking it to odd-chain fatty-acid metabolism, propionate handling, and the methylation cycle that regenerates tetrahydrofolate and methionine.
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.
Why is the B12 test ordered?
- Investigation of macrocytic anaemia, particularly when mean cell volume is elevated and the blood film shows macro-ovalocytes or hypersegmented neutrophils
- Evaluation of unexplained peripheral neuropathy, paraesthesiae, ataxia, or cognitive decline, especially in older adults
- Screening of patients with documented or suspected pernicious anaemia, atrophic gastritis, or positive intrinsic-factor or parietal-cell antibodies
- Baseline and periodic monitoring of strict vegans and vegetarians not taking B12 supplements
- Assessment of patients on long-term metformin, proton-pump inhibitors, or H2-receptor antagonists, which impair B12 absorption
- Work-up of malabsorption syndromes including coeliac disease, Crohn disease affecting the ileum, chronic pancreatitis, or post-gastrectomy or post-bariatric-surgery states
- Follow-up of equivocal or low-normal results when clinical suspicion remains high, interpreted with methylmalonic acid and homocysteine
- Patients with unexplained elevated homocysteine or methylmalonic acid detected on other testing
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
1–2 business days
Collection notes
Phlebotomy at the LabCorp patient service centre. Where baseline status rather than peak levels is required, advise withholding B12 supplementation for an appropriate interval prior to collection. Document metformin and acid-suppressing therapy.
Specimen requirements
Minimum 0.5 mL serum in a red-top or serum separator tube. Protect from light; B12 is photosensitive. Separate and refrigerate or freeze promptly if not analysed within a few hours.
What can affect B12 results?
Supplementation raises serum B12, sometimes to supraphysiological levels, masking underlying deficiency if sampled shortly after a dose; withholding supplements for several days to weeks before testing is advised when baseline status is the goal.
Metformin impairs ileal B12-intrinsic-factor uptake; proton-pump inhibitors and H2 antagonists reduce gastric acid and impair liberation of B12 from food, both leading to gradual depletion over months to years of therapy.
Nitrous oxide irreversibly oxidises the cobalt in B12, inactivating methionine synthase; acute recreational or anaesthetic exposure can precipitate acute deficiency with neurological sequelae even when baseline stores are adequate.
Red cells contain minimal B12, but haemolysis can interfere with some immunoassays; a grossly haemolysed sample should be rejected and redrawn.
Hepatocyte necrosis releases stored cobalamin; myeloproliferative disorders increase haptocorrin synthesis, raising total B12 while the metabolically active transcobalamin-bound fraction may remain normal or low.
Severe folate deficiency can mask B12 deficiency haematologically (both cause macrocytosis), and folate repletion without addressing concurrent B12 deficiency risks precipitating or worsening neurological damage; the two vitamins should be assessed together.
What do B12 results mean?
Elevated
Elevated B12 (>900 pg/mL) most commonly reflects recent supplementation. Persistently high values without supplementation raise concern for liver disease (acute hepatitis, cirrhosis), myeloproliferative or myelodysplastic disorders, or rarely a haptocorrin-producing tumour. Very high B12 in hospitalised patients has been associated with increased mortality, though the mechanism is not fully understood and may reflect underlying disease severity.
Normal
A result within the reference interval generally excludes deficiency, but values in the lower half (200–400 pg/mL) accompanied by neurological symptoms, macrocytosis, or elevated MMA/homocysteine may still represent functional deficiency. Interpret in the full clinical context rather than as an isolated number.
Reduced
Low B12 (<200 pg/mL) indicates deficiency until proved otherwise and warrants investigation of the cause: dietary (vegan diet without supplementation), gastric (pernicious anaemia, atrophic gastritis, gastrectomy), intestinal (ileal Crohn disease, resection, bacterial overgrowth, coeliac disease, pancreatic insufficiency), or drug-related (metformin, acid suppression). When B12 is low-normal (200–300 pg/mL) and clinical suspicion persists, measure methylmalonic acid and homocysteine; both rise in tissue deficiency. Initiate replacement and monitor haematological and neurological response.
Note: Serum B12 measures total cobalamin, much of which is bound to haptocorrin and not metabolically active. Equivocal results require correlation with MMA and homocysteine. A normal B12 does not exclude recent nitrous oxide inactivation. In patients with combined folate and B12 deficiency, treating folate alone can worsen B12 neuropathy. Serial monitoring after replacement should show normalisation within weeks to months; persistent elevation of MMA despite rising B12 suggests continued cellular deficiency or renal impairment (MMA is renally cleared).
Frequently asked questions
Serum B12 is the first-line screen measuring total circulating cobalamin, much of it bound to haptocorrin. Methylmalonic acid rises early when cellular B12 is insufficient and serves as the confirmatory test when B12 is equivocal but clinical suspicion remains high.
How to access the B12 test
The B12 reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: October 7, 2026
Last reviewed: October 7, 2026
- Devalia V, et al. British Committee for Standards in Haematology. Guidelines for the diagnosis and treatment of cobalamin and folate disorders. Br J Haematol. (2014)
- Green R, et al. Vitamin B12 deficiency. Nat Rev Dis Primers. (2017)
- Stabler SP. Vitamin B12 deficiency. N Engl J Med. (2013)
- Langan RC, Goodbred AJ. Vitamin B12 deficiency: recognition and management. Am Fam Physician. (2017)
- Carmel R. How I treat cobalamin (vitamin B12) deficiency. Blood. (2008)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.