Lab marker
Vitamin B12
Cobalamin · Cbl
Essential cofactor for DNA synthesis and methylation — deficiency is common, often subtle, and slowly reversible.
What it measures
Serum cobalamin concentration. Captures the bulk of circulating B12 (most bound to haptocorrin; only ~20% bound to transcobalamin and biologically active). Reported in pmol/L (SI) or pg/mL (US conversion: 1 pmol/L ≈ 1.357 pg/mL).
Reference context
3 guideline sources
Serum B12 has imperfect sensitivity for true deficiency — some patients with neurological deficiency have 'normal' serum B12. MMA and homocysteine become useful when symptoms suggest deficiency despite a borderline value.
Population context — consult guideline targets below
Mechanism
Why moving this marker matters
B12 is a cofactor for methionine synthase (methylation reactions, including DNA synthesis) and methylmalonyl-CoA mutase (fatty acid metabolism). Deficiency causes megaloblastic anaemia, peripheral neuropathy, cognitive impairment, and elevated homocysteine. Pernicious anaemia (autoimmune intrinsic factor deficiency) and metformin/PPI-induced malabsorption are common adult causes.
Guideline targets
What major guidelines recommend
BCSH 2014 (deficient)
<148 pmol/L (<200 pg/mL)
BCSH 2014 (borderline)
148–221 pmol/L (200–300 pg/mL) — interpret with MMA / homocysteine
BCSH 2014 (replete)
>221 pmol/L (>300 pg/mL)
How to measure
The test, where to get it, when to repeat
Method
Standard blood draw. Serum cobalamin is first-line; methylmalonic acid (MMA) and homocysteine are second-line confirmation when results are borderline. Holotranscobalamin ('active B12') is an emerging alternative measure of biologically available B12.
Where
GP standard panel or private lab.
Typical cost
€15–30 private; bundled in many comprehensive panels.
Fasting
Not required
When to test
BCSH 2014 (Devalia)
Test in adults with unexplained anaemia, macrocytosis, neurological symptoms, glossitis, or risk factors (vegan/vegetarian diet, metformin or PPI use, autoimmune conditions, prior gastrointestinal surgery).
NICE NG239 (UK 2024)
Updated 2024 guideline on B12 deficiency diagnosis and management.
Where to test
Independent labs offering this test
Healicus refers you to independent laboratories. You order from the lab; they take the sample, run it, and return your result on their own platform. Healicus never sees your value.
Randox Health
UK · EU · INTLClinic-based premium panels — wider biomarker breadth than home-test brands.
Visit Randox Health
Synlab
DE · EU · INTLEurope-wide medical lab network — referrals via partner GPs and direct-to-consumer programmes where offered.
Visit Synlab
Cerascreen
DE · EUEstablished German home-test catalogue — ISO-certified labs, German-language reports.
Visit Cerascreen
Medichecks
UKUKAS-accredited home blood-test panels with GP-equivalent biomarker coverage.
Visit Medichecks
Healicus is not the provider. Your contract for the service is with whoever you choose. Links labelled Sponsored are paid affiliate relationships; unlabelled links are editorial reference only. See our disclosure for the full policy.
Context
Reading the numbers
Serum B12 has imperfect sensitivity for true deficiency — some patients with neurological deficiency have 'normal' serum B12. MMA and homocysteine become useful when symptoms suggest deficiency despite a borderline value.
Caveats
Recent B12 supplementation falsely elevates serum cobalamin. Pregnancy lowers values transiently. Folate deficiency can mask the haematological signs of B12 deficiency while neurological harm proceeds — replete both when treating.
Practices
What's been shown to influence this marker
Mediterranean dietary pattern includes fish, dairy, and modest meat — adequate dietary B12 sources for non-vegan adults. Strict plant-only diets are an independent deficiency risk requiring supplementation.
Mediterranean dietary pattern
Habit·Olive oil, fish, nuts, legumes, plants. The most-studied diet for cardiovascular and cognitive longevity.
Why
The Mediterranean pattern — heavy on plants, olive oil, fish, nuts, legumes; moderate fish and dairy; light on red meat — has the strongest evidence base of any specific diet for long-term cardiovascular and cognitive outcomes. PREDIMED, the largest trial, showed ~30% reduction in major cardiovascular events vs. low-fat control.
Slot in your day
How to do it
How
Olive oil as the primary fat. Plants at every meal. Fish 2–3× per week. Nuts daily (small handful). Red meat once a week or less. Wine optional, with food.
Sticking with it
Stock the kitchen for one week's pattern. Decisions live in the shopping list, not at mealtime.
Markers this may influence
Evidence
See also
Related markers
Take to your physician
Worth discussing
- Whether borderline values warrant MMA or holotranscobalamin testing.
- If deficient, the likely cause (dietary, autoimmune pernicious anaemia, malabsorption, medication) and whether oral or intramuscular replacement is appropriate.
- When to retest after starting replacement (typically 3–6 months).
Sources
Cited literature
- [1]Devalia et al., BCSH — Guidelines for the diagnosis and treatment of cobalamin and folate disorders (Br J Haematol)(2014)
- [2]NICE NG239 — Vitamin B12 deficiency in over 16s: diagnosis and management(2024)
- [3]Pawlak et al., The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12 (Eur J Clin Nutr)(2014)
Edited by Carl Pöhl, MD · Healicus editorial
Last reviewed May 2026
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