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Vitamin B12 Test: What Your Level Means and Why Deficiency Matters

B12 deficiency causes megaloblastic anaemia and irreversible nerve damage. It's common, often missed, and entirely treatable: but early detection matters.1

Written and clinically reviewed by Suman Konda, PharmD, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: · How we check our content

Normal range
150–900 pmol/L (lab-dependent)
Deficiency
<150 pmol/L
Grey zone
150–300 pmol/L: interpret with symptoms
Nerve damage risk
Irreversible if prolonged

Why B12 Matters

  • DNA synthesis: required for all dividing cells
  • Myelin synthesis: protects nerves; deficiency causes subacute combined degeneration of the cord
  • Homocysteine metabolism: high homocysteine damages blood vessels
  • Red cell production: deficiency causes macrocytic anaemia
  • Neurological function: deficiency causes memory loss, tingling, ataxia

Causes of B12 Deficiency

CauseMechanismPopulation at Risk
Pernicious anaemiaAutoimmune loss of intrinsic factor → no B12 absorptionMiddle-aged, Northern European, autoimmune family history
Vegan / vegetarian dietB12 only in animal products: no dietary sourceVegans without supplementation
Metformin useReduces B12 absorption in terminal ileumDiabetics on metformin for >2 years
Gastric surgery / bariatricLoss of parietal cells → reduced IF and acidPost-gastrectomy, sleeve gastrectomy
Crohn's disease (terminal ileum)Absorption site damagedCrohn's with ileal involvement
PPIs (proton pump inhibitors)Reduced acid impairs B12 release from foodLong-term PPI users
Check Active B12Standard B12 measures total B12, including inactive forms. Holotranscobalamin (active B12) or methylmalonic acid (MMA) are more sensitive markers of functional deficiency, especially in the grey zone.
Neurological Symptoms Are UrgentB12 deficiency causing subacute combined degeneration of the cord (tingling, leg weakness, unsteadiness, cognitive decline) requires urgent replacement: nerve damage can become permanent.
Can B12 be normal on a blood test but still be deficient?
Yes. Total B12 can be normal if inactive forms are high. Check holotranscobalamin (active B12) or MMA if symptoms suggest deficiency despite borderline normal results.
How is B12 deficiency treated?
Pernicious anaemia or malabsorption: intramuscular B12 injections (hydroxocobalamin), typically loading doses then every 3 months. Dietary deficiency: high-dose oral supplements (1000 µg/day, overcomes poor absorption by passive diffusion).
Can vegetarians get enough B12?
Not through diet alone: B12 is found only in animal products (meat, fish, eggs, dairy). Vegetarians who eat dairy/eggs may get enough; vegans must supplement. Fortified foods (plant milks, nutritional yeast) help but may be insufficient.
Does B12 deficiency cause depression?
B12 is involved in serotonin and dopamine synthesis. Deficiency is associated with depression, cognitive impairment, and fatigue. Correcting deficiency often improves mood alongside other symptoms.

Frequently asked questions

Why does my lab's Vitamin B12 range differ from the one shown here?
Every laboratory sets its own reference interval from the analyser, reagents and reference population it uses, so two labs can publish different Vitamin B12 intervals and both be correct, even reporting in the same pmol/L. Read your result against the range printed on your own report.
Is a slightly abnormal Vitamin B12 a reason to worry?
Rarely on its own. Reference intervals are built to contain the middle 95% of a healthy population, so mild deviations are common in well people. The size of the deviation, its direction over time and your symptoms decide whether it means anything.
Why did my Vitamin B12 change when nothing else did?
Biological variation is real. Hydration, time of day, recent food, exercise and a recent infection all move Vitamin B12 to some degree, and every assay carries its own small imprecision on top of that.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

  1. Vitamin B12 Deficiency. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK441923

References

Sources cited on this page. PubMed links open the original abstract.

  1. Stabler SP. Vitamin B12 deficiency. N Engl J Med. 2013;368(2):149–160. PMID 23301732 · doi:10.1056/NEJMcp1113996

Why low vitamin B12 is often missed – and who is most at risk

Vitamin B12 deficiency is estimated to affect 6% of adults under 60 and nearly 20% of those over 60 in Western countries, yet it frequently goes undiagnosed because symptoms develop insidiously over years. The main at-risk groups are: older adults (due to gastric atrophy reducing intrinsic factor secretion); vegans and strict vegetarians (B12 occurs almost exclusively in animal products); patients taking metformin (which reduces B12 absorption over years, affecting up to 30% of long-term users); patients on proton pump inhibitors (PPI) for more than 2 years; and anyone with autoimmune gastritis (pernicious anaemia), Crohn's disease, or following gastrointestinal surgery including bariatric procedures.

Understanding the B12 result – active B12 versus total B12

The standard serum B12 test measures total B12 – including both active and inactive forms. About 80% of circulating B12 is bound to haptocorrin (biologically inactive transport protein), and only 20% is bound to transcobalamin II and metabolically active. This means total B12 can appear normal while functional deficiency exists. Holotranscobalamin (active B12) is a more sensitive and specific marker – laboratories may measure it when total B12 is borderline (150–300 pmol/L). Methylmalonic acid (MMA) and homocysteine are functional markers that rise when cellular B12 is truly insufficient: elevated MMA specifically reflects B12 deficiency, while elevated homocysteine is seen in both B12 and folate deficiency.

The neurological emergency – subacute combined degeneration of the cord

Severe B12 deficiency can cause an irreversible neurological syndrome called subacute combined degeneration of the spinal cord (SACD). This involves degeneration of the dorsal and lateral columns of the spinal cord, causing progressive peripheral neuropathy (glove-and-stocking numbness and tingling), then loss of proprioception and vibration sense, then spastic weakness of the legs. If untreated, it progresses to paraplegia. Critically, neurological involvement can occur without anaemia – the absence of macrocytic anaemia does not rule out B12-related nerve damage. Any patient with unexplained peripheral neuropathy or sensory ataxia should have B12 checked. Treatment is intramuscular hydroxycobalamin injections, which prevent further progression but may not reverse established damage.

Related reading

Medical Disclaimer: This page is for general education only and does not replace professional medical advice. Always consult a qualified healthcare provider.
Content written and reviewed by Suman Konda, PharmD, Clinical Pharmacist · Telangana State Pharmacy Council · Sources linked to PubMed · Not medical advice – see our disclaimer