Vitamin B12 (Cobalamin), Serum
The Vitamin B12 Serum test measures total cobalamin concentration in serum as the primary first-line screening marker for B12 deficiency. Serum B12 below 200 pg/mL confirms deficiency; values between 200 and 400 pg/mL represent a borderline zone where functional deficiency is present in approximately 25–35% of patients and requires confirmation by methylmalonic acid and homocysteine.
1
Biomarkers
Blood (serum)
Sample
1–3 business days
Turnaround
No — fasting is not required for serum vitamin B12. Recent dietary intake does not materially affect serum B12 due to the complexity of the absorption pathway and the large hepatic storage pool. However: (1) document all B12 supplementation — oral dose, frequency, and date of most recent dose — as high-dose supplementation substantially elevates serum B12 and requires a washout context for accurate baseline assessment; (2) document intramuscular B12 injection timing — the date and dose of the most recent injection must be recorded for results to be interpretable.
Fasting
What is the Vitamin B12 test?
The Vitamin B12 Serum test quantifies total cobalamin — including haptocorrin-bound and transcobalamin II-bound fractions — in serum, providing the primary screening measurement of B12 status in clinical practice. It is ordered across a broad range of clinical indications: as a first-line nutritional deficiency screen when dietary intake inadequacy is suspected; as a mandatory component of macrocytic and megaloblastic anaemia investigation; as a prerequisite before folate supplementation in patients with macrocytic anaemia; in the evaluation of neurological symptoms including paraesthesia, balance disturbance, cognitive decline, and peripheral neuropathy where B12-related demyelination is a differential; in population screening of high-risk groups including vegetarians, vegans, elderly patients, and those on long-term acid-suppressing medications; and as a monitoring marker in patients receiving oral or parenteral B12 supplementation. The most important interpretive principle for serum B12 is its significant false-negative rate at concentrations between 200 and 400 pg/mL — a zone in which patients with genuine functional B12 deficiency (elevated MMA and elevated homocysteine) are frequently classified as 'normal' by serum B12 alone. This limitation arises because serum B12 measures total circulating cobalamin including the metabolically inactive haptocorrin-bound fraction, which may be elevated by liver disease, haematological conditions, and solid tumours independently of cellular B12 adequacy. When serum B12 is in the borderline range in a symptomatic patient, or when neurological or macrocytic features persist despite a 'normal' serum B12, methylmalonic acid and total homocysteine measurement provides the functional confirmation of cellular B12 sufficiency that serum B12 cannot deliver — MMA is a highly specific marker that rises as soon as cellular adenosylcobalamin (the mitochondrial B12 cofactor) is depleted, often weeks to months before serum B12 falls below 200 pg/mL.
What does the Vitamin B12 test measure?
Vitamin B12 is a water-soluble, cobalt-containing organometallic vitamin available exclusively from animal-source foods — meat, poultry, fish, shellfish, eggs, and dairy — and from fortified foods or supplementation. The absorption pathway is uniquely complex among vitamins: dietary B12 is released from food protein by gastric acid and pepsin, binds salivary haptocorrin (R-protein) in the stomach, is released from haptocorrin by pancreatic proteases in the duodenum, and then binds intrinsic factor (IF) — a 44 kDa glycoprotein secreted by gastric parietal cells — forming an IF-B12 complex that binds cubilin receptors in the terminal ileum for endocytosis. This multi-step absorption process creates multiple vulnerability points: atrophic gastritis impairs gastric acid and pepsin release; proton pump inhibitors reduce acid-dependent food-B12 liberation; total or subtotal gastrectomy eliminates intrinsic factor production; ileal disease or resection eliminates the absorption site; and pancreatic insufficiency impairs haptocorrin digestion. In the circulation, B12 is transported bound to haptocorrin (transcobalamin I — approximately 80%, a reservoir fraction with low cellular uptake) and transcobalamin II (approximately 20% — the metabolically active holotranscobalamin fraction that delivers B12 to all cells). B12 serves as a cofactor for two critical enzymatic reactions: methionine synthase (converting homocysteine to methionine using 5-methyltetrahydrofolate as the methyl donor — the reaction connecting the B12 and folate metabolic cycles) and methylmalonyl-CoA mutase (converting methylmalonyl-CoA to succinyl-CoA in the mitochondria, essential for odd-chain fatty acid and amino acid catabolism). Total body stores of 2–5 mg — predominantly held in the liver — support 2–4 years of dietary absence before clinical deficiency manifests, explaining the delayed presentation of B12 deficiency following dietary change or absorption pathway disruption.
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.
Why is the Vitamin B12 test ordered?
- Macrocytic anaemia investigation — serum B12 is the mandatory first-line test when MCV exceeds 100 fL, with concurrent folate measurement, to identify megaloblastic nutritional deficiency before treatment decisions are made; folate therapy initiated without first confirming B12 status risks masking haematological B12 deficiency while neurological disease progresses
- Peripheral neuropathy and neurological symptom evaluation — B12 in the assessment of paraesthesia, numbness, balance disturbance, proprioceptive loss, cognitive impairment, and psychiatric symptoms where subacute combined degeneration of the spinal cord or B12-related demyelination is a plausible differential alongside other causes
- Vegan, vegetarian, and plant-based dietary pattern screening — dietary B12 is found exclusively in animal-sourced foods; vegans and strict vegetarians are at near-universal risk of B12 deficiency without supplementation, and baseline and annual monitoring serum B12 is recommended regardless of symptom status
- Long-term proton pump inhibitor (PPI) and H2 blocker use — chronic acid suppression impairs the gastric acid-dependent release of protein-bound dietary B12, producing progressive B12 depletion over 2–5 years of continuous use; PPI users require periodic B12 monitoring, particularly those on PPI therapy for more than 2 years
- Elderly patient nutritional and cognitive assessment — atrophic gastritis — affecting up to 30% of individuals over 65 years — impairs both gastric acid and intrinsic factor production, progressively reducing B12 absorption; B12 deficiency is a reversible contributor to cognitive impairment and dementia-like presentations in elderly patients and warrants exclusion in any new cognitive assessment
- Post-gastric surgery and bariatric surgery monitoring — total or subtotal gastrectomy eliminates intrinsic factor production; Roux-en-Y gastric bypass bypasses the proximal stomach and alters gastric acid secretion; both procedures mandate lifelong B12 monitoring and parenteral or high-dose oral supplementation
- Malabsorption syndrome and gastrointestinal disease evaluation — ileal Crohn's disease, coeliac disease, tropical sprue, short bowel syndrome, and pancreatic exocrine insufficiency all impair B12 absorption through distinct mechanisms; B12 screening is a standard nutritional assessment component in all these conditions
- Monitoring response to B12 supplementation — serum B12 to confirm therapeutic levels following initiation of oral cyanocobalamin, methylcobalamin, or intramuscular hydroxocobalamin supplementation, ensuring adequate repletion in the context of confirmed or suspected deficiency
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No — fasting is not required for serum vitamin B12. Recent dietary intake does not materially affect serum B12 due to the complexity of the absorption pathway and the large hepatic storage pool. However: (1) document all B12 supplementation — oral dose, frequency, and date of most recent dose — as high-dose supplementation substantially elevates serum B12 and requires a washout context for accurate baseline assessment; (2) document intramuscular B12 injection timing — the date and dose of the most recent injection must be recorded for results to be interpretable.
Collection method
Venipuncture at patient service centre
Turnaround
1–3 business days
Collection notes
Standard venipuncture into a serum separator tube (SST / gold top). Optimal venepuncture technique is important for serum B12 — haemolysis releases erythrocyte B12 into serum and is the most common cause of spuriously elevated results masking true deficiency. Minimise tourniquet time, use an appropriately sized needle, gently invert the SST 5 times only, and centrifuge within 2 hours of collection. Visually inspect separated serum for haemolysis before shipping — pink or red discolouration indicates haemolysis and the specimen must be recollected. Document on the requisition form: (1) current oral or parenteral B12 supplementation and most recent dose; (2) dietary pattern — vegan, vegetarian, or animal-product-restricted; (3) relevant medications — metformin dose and duration, PPI or H2 blocker use and duration, colchicine, anticonvulsants; (4) gastrointestinal or surgical history — gastrectomy, bariatric surgery, ileal Crohn's disease, coeliac disease, pancreatic insufficiency; (5) nitrous oxide anaesthetic exposure in the preceding 4 weeks; (6) pregnancy status and trimester; (7) relevant clinical features — macrocytosis on prior CBC, neurological symptoms, cognitive symptoms. # REVIEWER FLAG: Confirm Access Medical Laboratories SST tube type, minimum serum volume, maximum time from collection to centrifugation, and cold-chain shipping requirements for vitamin B12 before publication.
Specimen requirements
Minimum 0.5 mL serum. Centrifuge and separate serum from cells within 2 hours of collection — prompt separation is important to minimise any erythrocyte B12 leakage. Stable at 2–8°C for up to 7 days; stable frozen at −20°C for extended storage. Protect from prolonged light exposure. # REVIEWER FLAG: Confirm minimum volume and stability data with Access Medical Laboratories assay documentation.
What can affect Vitamin B12 results?
Erythrocytes contain substantial intracellular vitamin B12 — approximately 10–12 times the serum concentration — bound to haptocorrin. In vitro haemolysis during or after phlebotomy releases erythrocyte B12 into serum, producing a spuriously elevated total serum B12 that can mask true deficiency by raising an otherwise deficient result into the reference range. Even visually imperceptible haemolysis — detectable only by the analyser's haemolysis index — can artefactually elevate serum B12 by a clinically significant amount. Haemolysed specimens must be rejected and recollected for accurate B12 assessment. This is the most common laboratory artefact causing false-normal B12 results.
High-dose oral B12 (≥500 µg/day cyanocobalamin or methylcobalamin) or recent intramuscular hydroxocobalamin injection produces a rapid and substantial rise in serum B12, often to values well above 900 pg/mL, reflecting supplement exposure rather than habitual nutritional status or tissue stores. Pre-treatment baseline serum B12 assessment must be performed before supplementation initiation. If monitoring on-treatment repletion, collection timing relative to the most recent dose must be documented — weekly intramuscular injection protocols produce a peak-trough variation that requires consistent timing for interpretable serial comparison.
Hepatic disease — including acute hepatitis, cirrhosis, and hepatocellular carcinoma — causes release of hepatic B12 stores and elevated haptocorrin synthesis, producing spuriously elevated serum B12 that does not reflect adequate cellular B12 status. Haematological malignancies — particularly chronic myeloid leukaemia, polycythaemia vera, and certain myeloproliferative neoplasms — produce markedly elevated serum B12 (often above 2000 pg/mL) through increased haptocorrin production by the expanded granulocyte pool. Unexpectedly elevated serum B12 above 1000–2000 pg/mL without supplementation should prompt clinical investigation for underlying liver disease or myeloproliferative disorder.
Nitrous oxide irreversibly oxidises the cobalt centre of cobalamin (methylcobalamin) from its active Co(I) form to the inactive Co(III) form, permanently inactivating methionine synthase and precipitating acute functional B12 deficiency — even in patients with apparently normal or adequate serum B12 concentrations. This occurs within hours of N₂O exposure and has caused acute neurological deterioration and megaloblastic bone marrow change in patients with subclinical B12 deficiency who underwent N₂O anaesthesia. Patients with borderline or low serum B12 should be flagged before any procedure involving N₂O anaesthesia.
Oral contraceptive use is associated with reduced serum B12 through multiple proposed mechanisms including altered B12 binding protein concentrations and increased B12 utilisation by hormonally stimulated tissues. Pregnancy substantially increases B12 demand for foetal neurological development and placental requirements, progressively lowering maternal serum B12 — particularly in the second and third trimesters — even in women with adequate pre-pregnancy stores. Pregnancy-specific B12 reference ranges should be applied, and B12 screening is a standard component of antenatal nutritional assessment. # REVIEWER FLAG: Confirm whether Access Medical Laboratories provides pregnancy-specific reference ranges for vitamin B12 when pregnancy is flagged on the requisition form.
Metformin — the most widely prescribed oral antidiabetic agent — reduces serum B12 through calcium-dependent ileal cubilin receptor interference that impairs IF-B12 complex absorption, producing a 10–30% reduction in serum B12 over time. The prevalence of B12 deficiency in long-term metformin users is approximately 5–10 times higher than in non-users. Other medications that impair B12 status include colchicine (ileal IF-B12 absorption inhibition), cholestyramine (bile acid sequestration altering enterohepatic B12 recirculation), and chronic high-dose anticonvulsant use. Metformin use and duration must be documented on the requisition form.
What do Vitamin B12 results mean?
Elevated
Serum B12 above 900 pg/mL in the absence of documented supplementation or injection warrants clinical investigation — unexplained elevated B12 may indicate hepatic disease with hepatic B12 release, myeloproliferative neoplasm with increased haptocorrin from expanded granulocyte mass (CML, polycythaemia vera, essential thrombocythaemia), solid tumour with haptocorrin overproduction, or chronic myeloid leukaemia. Values above 1500–2000 pg/mL without supplementation context should prompt haematological and hepatic clinical review. In the context of confirmed supplementation, elevated B12 merely reflects recent dose exposure and is not clinically significant from a toxicity standpoint — unlike fat-soluble vitamins, B12 does not accumulate to toxic levels in tissue under normal physiological conditions.
Normal
Serum B12 within the reference range (200–900 pg/mL) provides reassurance against frank B12 deficiency in the absence of haemolysis or the confounders described above. In a symptomatic patient — particularly one with neurological features, macrocytosis, or risk factors for functional B12 deficiency (PPI use, metformin, vegan diet, elderly age) — a 'normal' serum B12 between 200 and 400 pg/mL should not be taken as definitive exclusion of B12 deficiency without functional marker confirmation. In functional medicine, the target is the upper half of the reference range (400–900 pg/mL) for adequate methylation and neurological reserve — patients in the 200–400 pg/mL range are flagged for dietary, supplementation, or functional assessment regardless of whether they cross the formal deficiency threshold.
Reduced
Serum B12 below 200 pg/mL confirms deficiency and mandates treatment. The urgency of treatment depends on the clinical syndrome: macrocytic anaemia without neurological features may be managed with high-dose oral supplementation in patients without absorption pathway defects; neurological features — paraesthesia, ataxia, cognitive decline — require parenteral hydroxocobalamin (intramuscular) regardless of oral absorption capacity, given the urgency of achieving adequate tissue B12 rapidly to prevent irreversible neurological damage. Serum B12 in the borderline range (200–400 pg/mL) in a symptomatic patient warrants functional confirmation with methylmalonic acid and total homocysteine — elevated MMA (>0.4 µmol/L) with or without elevated homocysteine confirms cellular B12 deficiency at a tissue level despite a 'normal' serum B12. Concurrent CBC review for macrocytosis and hypersegmented neutrophils and folate assessment are mandatory alongside a deficient or borderline B12 result.
Note: The principal interpretive caveat is the sensitivity gap of serum B12 in the 200–400 pg/mL borderline range. Studies using methylmalonic acid as the functional gold standard have shown that approximately 25–35% of patients with serum B12 in the 200–400 pg/mL range have elevated MMA — indicating true functional cellular B12 deficiency — that would be missed by serum B12 interpretation alone. Conversely, serum B12 may appear artificially elevated — masking deficiency — due to haemolysis, liver disease, or myeloproliferative disorders elevating haptocorrin-bound B12. A second important caveat is that holotranscobalamin (holoTC, also called Active B12) — the transcobalamin II-bound metabolically available fraction — is the superior marker of cellular B12 delivery and can detect depletion earlier than total serum B12. While holoTC assays are not universally available through standard laboratory networks, they may be the preferred assessment when early functional deficiency detection is the clinical priority. Thirdly: B12 deficiency neurological disease can occur without haematological abnormality — practitioners should never use the absence of macrocytic anaemia to exclude B12 deficiency in a patient with neurological symptoms.
Frequently asked questions
The serum vitamin B12 test measures total cobalamin — including both the haptocorrin-bound storage fraction (approximately 80%) and the transcobalamin II-bound metabolically active fraction (approximately 20%) — providing the primary first-line assessment of B12 nutritional status. Serum B12 below 200 pg/mL confirms deficiency; values between 200 and 400 pg/mL represent a borderline zone where functional deficiency may be present despite a technically normal result, requiring methylmalonic acid and homocysteine for confirmation.
How to access the Vitamin B12 test
The Vitamin 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: May 24, 2026
Last reviewed: May 24, 2026
- Stabler SP. Clinical practice. Vitamin B12 deficiency. N Engl J Med. (2013)
- Carmel R. How I treat cobalamin (vitamin B12) deficiency. Blood. (2008)
- Langan RC, Goodbred AJ. Vitamin B12 Deficiency: Recognition and Management. Am Fam Physician. (2017)
- Andrès E, et al. Vitamin B12 (cobalamin) deficiency in elderly patients. CMAJ. (2004)
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.
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