Vitamins and nutrition . Patient guide
Vitamin B12 (B12)
Also known as: Cobalamin, Cyanocobalamin
Vitamin B12 is an essential nutrient for red blood cell formation, neurological function and DNA synthesis. The blood B12 level (most commonly measured as serum cobalamin or total B12) is the standard first-line test for deficiency.
This biomarker entry is being clinically reviewed by our team. The factual content draws on UK guidance (NICE, NHS, Royal Colleges and the relevant speciality society where cited).
Reference range
Reported in ng/L (also seen as pmol/L (varies by lab)). Final reports always carry the issuing laboratory's range, which is what your clinician will interpret against.
| Group | Range | Note |
|---|---|---|
| Deficient | under 200 | ng/L (or pmol/L equivalent, depending on lab) |
| Borderline (NICE: 200 to 350) | 200 to 350 | |
| Adequate | 350 to 900 | |
| Symptomatic patients (clinical view) | aim above 400 |
What it is
B12 is found almost exclusively in animal products. It is absorbed in the lower small intestine after binding to intrinsic factor produced by the stomach. Both diet and absorption matter, so deficiency can occur from either route.
Why a clinician would order it
Common reasons include persistent fatigue, tingling or numbness in hands and feet, balance problems, sore or red tongue, hair shedding, low mood or memory problems, vegan or strictly vegetarian diet, long-term metformin use, long-term proton-pump-inhibitor (PPI) use, or known gastric or bowel surgery.
If your level is outside the range
Symptoms of low B12
- Persistent fatigue
- Hair shedding
- Pale skin or yellow tinge
- Sore, red tongue
- Tingling in hands or feet
- Memory or concentration problems
- Balance problems
What low can indicate. Dietary insufficiency (vegan, vegetarian), pernicious anaemia (autoimmune loss of intrinsic factor), metformin and PPI use over years, coeliac or Crohn disease, post-gastric-surgery malabsorption.
Symptoms of high B12
- Usually no symptoms; can be raised in liver disease or some blood disorders
What high can indicate. Recent supplementation or injection. Persistently high levels can occasionally indicate liver disease or myeloproliferative disorders and warrant clinical review.
Testing tips
No fasting required. Where possible, stop B12 supplements for at least 4 weeks before testing for a true baseline (only with your prescriber’s agreement). Borderline results often warrant a follow-up test for active B12 (holotranscobalamin) or methylmalonic acid (MMA).
Where you can get this tested
Vitamin B12 is included in the following WMG Health panels. Same-day appointments at our Harley Street clinic, with results clinician-reviewed.
Want a specific combination of markers we do not have a panel for? Build a custom panel and our clinicians will design one for you.
Sources
UK guidance our clinicians use when interpreting this marker.
This page is general patient information, not personal medical advice. A GMC-registered clinician will review your results and tailor any interpretation to you. See our Editorial Policy for how we write and review content.