Vitamins and nutrition . Patient guide
Vitamin B12 Blood Test
Also known as: Cobalamin, Cyanocobalamin
What is B12
Vitamin B12 (cobalamin) is an essential nutrient for red blood cell formation, neurological function and DNA synthesis, and a serum B12 (total cobalamin) is the standard first-line test for deficiency. According to NICE NG239 (March 2024) Table 1, a serum total B12 below 180 ng/L (133 pmol/L) is diagnostic of deficiency, 180 to 350 ng/L is the indeterminate zone where active B12 (holotranscobalamin) or methylmalonic acid (MMA) helps clarify, and above 350 ng/L makes deficiency unlikely. B12 deficiency is particularly common in vegan and vegetarian diets, in adults over 50 due to gastric atrophy, and in patients on long-term metformin or proton pump inhibitors.
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 |
|---|---|---|
| Confirmed deficiency (NICE NG239) | under 180 | ng/L, equivalent to under 133 pmol/L; NG239 Table 1 (March 2024) lowered the cut-off from the older WHO/BSH 200 ng/L figure to reduce overdiagnosis. ng/L and pmol/L are not numerically interchangeable (conversion factor approximately 1.355) |
| Indeterminate (NICE NG239) | 180 to 350 | NG239 Table 1: 133 to 258 pmol/L indeterminate zone; correlate with symptoms and consider active B12 (holotranscobalamin) or MMA |
| Adequate | 350 to 900 | |
| Symptomatic patients (clinical view) | often above 400 | expert opinion for patients with neurological or fatigue symptoms despite an in-range serum B12; not a NICE threshold |
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. IMPORTANT: stop oral B12 supplements (tablets, sprays, lozenges, multivitamins or B-complex containing B12) for several weeks before testing - ideally 4 weeks - for a true baseline. A serum B12 result taken on supplementation can read normal even when an absorption problem is masking a real deficiency, which is the single biggest pitfall in B12 testing. If you have had a B12 injection (hydroxocobalamin), serum levels can stay elevated for weeks to months and timing should be agreed with your prescriber. Borderline results often warrant a follow-up test for active B12 (holotranscobalamin) or methylmalonic acid (MMA), both of which are less affected by recent intake.
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.
Symptoms often investigated with Vitamin B12
Vitamin B12 is commonly tested when patients present with the following symptoms. If any of these resonate with you, the linked guides explain what to look for and which test pathway is appropriate.
Read this marker alongside another
Vitamin B12 is most useful when interpreted together with the markers below. Each guide walks through the 4-quadrant matrix our clinicians use when both come back at once.
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.
Common questions about B12
What is a normal B12 range?
Confirmed deficiency (NICE NG239): under 180 (ng/L, equivalent to under 133 pmol/L; NG239 Table 1 (March 2024) lowered the cut-off from the older WHO/BSH 200 ng/L figure to reduce overdiagnosis. ng/L and pmol/L are not numerically interchangeable (conversion factor approximately 1.355)). Indeterminate (NICE NG239): 180 to 350 (NG239 Table 1: 133 to 258 pmol/L indeterminate zone; correlate with symptoms and consider active B12 (holotranscobalamin) or MMA). Adequate: 350 to 900. Symptomatic patients (clinical view): often above 400 (expert opinion for patients with neurological or fatigue symptoms despite an in-range serum B12; not a NICE threshold). Always interpret your own results against the laboratory range printed on your report, since assay-specific reference ranges vary.
What does a low B12 result mean?
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.
What does a high B12 result mean?
Recent supplementation or injection. Persistently high levels can occasionally indicate liver disease or myeloproliferative disorders and warrant clinical review.
Do I need to fast or prepare for the B12 blood test?
No fasting required. IMPORTANT: stop oral B12 supplements (tablets, sprays, lozenges, multivitamins or B-complex containing B12) for several weeks before testing - ideally 4 weeks - for a true baseline. A serum B12 result taken on supplementation can read normal even when an absorption problem is masking a real deficiency, which is the single biggest pitfall in B12 testing. If you have had a B12 injection (hydroxocobalamin), serum levels can stay elevated for weeks to months and timing should be agreed with your prescriber. Borderline results often warrant a follow-up test for active B12 (holotranscobalamin) or methylmalonic acid (MMA), both of which are less affected by recent intake.
Can I order a B12 blood test privately in London?
Yes. WMG Health offers B12 as part of bespoke panels and several pre-built panels at our 134 Harley Street clinic. Results are clinician-reviewed by a GMC-registered doctor within 4 hours for the most common assays. All panels are custom-built around your specific question; bookings via /contact/ or 020 3239 3378.