What does a B12 blood test measure?
A serum vitamin B12 test (also called serum cobalamin) measures the total amount of B12 circulating in your blood. B12 is stored mainly in the liver, with enough reserve to last 3–5 years — which is why deficiency often develops slowly and is missed until symptoms become significant.
The test is typically ordered when investigating anaemia (particularly macrocytic anaemia, where red blood cells are abnormally large), neurological symptoms, fatigue, or to monitor people at risk of deficiency.
B12 reference ranges and interpretation
| Result | Category | What it generally means |
|---|---|---|
| < 150 pmol/L (< 200 pg/mL) | Deficient | Consistent with B12 deficiency; clinical symptoms likely or developing. |
| 150–220 pmol/L (200–300 pg/mL) | Borderline / Grey zone | Functionally deficient in some individuals; further testing recommended. |
| 221–700 pmol/L (300–950 pg/mL) | Normal | Adequate B12 levels for most adults. |
| > 700 pmol/L (> 950 pg/mL) | High (may warrant investigation) | Can occur with supplementation, liver disease, or myeloproliferative disorders. |
Units vary by laboratory — some report in pmol/L, others in pg/mL or ng/L (1 pmol/L ≈ 1.35 pg/mL). Always check which unit your lab uses and compare against the interval on your own report.
Symptoms of B12 deficiency
B12 deficiency has two main categories of consequence — haematological and neurological:
Blood-related symptoms
- Fatigue and weakness (often the first complaint)
- Pale or slightly yellow skin (jaundice from haemolysis of fragile red blood cells)
- Shortness of breath on exertion
- Macrocytic anaemia — enlarged red blood cells with lower oxygen-carrying capacity
Neurological symptoms
- Tingling or numbness in hands and feet (peripheral neuropathy)
- Balance problems and difficulty walking
- Memory difficulty, brain fog, or mood changes
- In severe, untreated cases: subacute combined degeneration of the spinal cord
Neurological damage from prolonged B12 deficiency can be irreversible. This is why treatment should not be delayed when deficiency is clinically suspected, even if the serum B12 is in the low-normal grey zone.
Common causes of low B12
- Pernicious anaemia — an autoimmune condition where the stomach fails to produce intrinsic factor, the protein needed to absorb B12. It is the most common cause of severe B12 deficiency in adults over 50.
- Vegan and vegetarian diets — B12 is found almost exclusively in animal products. Strict plant-based dieters require supplementation or fortified foods.
- Atrophic gastritis / reduced stomach acid — common in older adults; stomach acid is needed to release B12 from food. Long-term proton pump inhibitor (PPI) use contributes.
- Metformin use — the diabetes medication interferes with B12 absorption; B12 should be monitored annually in long-term metformin users.
- Gastrointestinal surgery — particularly gastric bypass or ileal resection, which reduce the absorptive surface for B12.
- Malabsorption conditions — coeliac disease, Crohn's disease, SIBO (small intestinal bacterial overgrowth).
What does a high B12 mean?
Elevated serum B12 is often dismissed, but a genuinely high result (not explained by recent supplementation) warrants attention. Causes include:
- B12 supplements or injections — by far the most common reason; transient and harmless
- Liver disease — the liver releases stored B12 when cells are damaged (hepatitis, cirrhosis)
- Myeloproliferative disorders — conditions like polycythaemia vera or chronic myeloid leukaemia can raise B12 significantly
- Solid tumours — some cancers increase B12 transport proteins, raising total serum B12
A high B12 without obvious explanation (e.g., no supplements) should be discussed with your doctor, particularly if you have other unexplained blood abnormalities.
Why the grey zone matters — and what functional testing adds
The borderline range (150–220 pmol/L or 200–300 pg/mL) is called the "grey zone" because some people in this range are functionally deficient while others are not. Two additional markers can clarify this:
- Methylmalonic acid (MMA) — rises specifically when B12-dependent cellular reactions fail. It is the most sensitive functional marker of B12 deficiency.
- Homocysteine — rises when both B12 and folate are insufficient. Less specific for B12 alone since folate, B6, and renal function also affect it.
Elevated MMA with low-normal B12 strongly suggests functional deficiency requiring treatment. Your doctor may also order intrinsic factor antibodies to test for pernicious anaemia.
B12 treatment: oral vs injection
High-dose oral B12 (typically 1000–2000 mcg/day) is as effective as intramuscular injections for most causes of deficiency — including pernicious anaemia — because passive diffusion can absorb around 1% of an oral dose without intrinsic factor. Injections are preferred when absorption is severely impaired or neurological symptoms are significant.
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