Vitamin B12
Also reported as: Cobalamin · Serum B12
Circulating cobalamin, most of which is bound to proteins that do not deliver it to tissues.
The lower bound is widely regarded as too low; deficiency symptoms occur between 200 and 400 pg/mL.
Neurological symptoms and elevated methylmalonic acid are found in people in the 200–400 pg/mL range, so a comfortable margin above that is the usual target.
To convert to pmol/L, multiply by 0.738.
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Where the thresholds sit
| Band | Range (pg/mL) | Note |
|---|---|---|
| Deficient | < 200 | — |
| Borderline | 200–400 | Confirm with methylmalonic acid if symptomatic. |
| Adequate | 400–500 | — |
| Optimal | 500–900 | — |
| High | > 900 | Usually supplementation; rarely liver or myeloproliferative disease. |
Why it matters
Deficiency causes neurological damage that can become irreversible if it persists, and the earliest signs are subtle — numbness, low mood, brain fog. The catch is that serum B12 is an insensitive test: symptomatic deficiency occurs at levels labs call normal, which is why the low-normal band deserves a second look rather than a shrug.
What raises it
- Supplementation
- Animal foods
- Liver disease
What lowers it
- Vegan and strict vegetarian diets
- Metformin
- Proton pump inhibitors and H2 blockers
- Pernicious anaemia
- Ageing — absorption declines with reduced stomach acid
- Nitrous oxide use
How to move it
- 1If you are vegan, supplement. This is not optional — there is no reliable plant source.
- 2If you take metformin or a PPI long term, have it checked periodically.
- 3Where the result is borderline and symptoms fit, ask for methylmalonic acid — it is the more sensitive test.
- 4Oral supplementation works for most people; injections are needed for pernicious anaemia and severe malabsorption.
What moves this marker
Interventions on this site with evidence behind them for this specific marker. None of them replaces working out why yours is where it is.
What makes a single reading misleading
- Serum B12 measures total cobalamin, much of it biologically unavailable. Active B12 (holotranscobalamin) is more informative.
- Folate supplementation can mask the haematological signs of B12 deficiency while neurological damage continues.
- A normal serum B12 does not exclude functional deficiency.
Sources
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Educational only. Reference ranges are not diagnostic thresholds and this page cannot account for your medical history, medication or symptoms. Discuss any result that concerns you with a qualified clinician. See our medical disclaimer.