Vitamin B12 blood test: what it measures and what can change it
Serum B12 measures total vitamin B12 in the blood; some laboratories measure active B12 instead.
General information about the test, from Hea's guide and the NHS · Last reviewed 5 November 2026
What it measures, and why GPs check it
Vitamin B12
Serum B12 measures total vitamin B12 in the blood; some laboratories measure active B12 instead.
Low B12 causes anaemia and nerve symptoms; NICE set the bands in 2024.
Active B12 (holotranscobalamin)
Active B12 (holotranscobalamin) is the part of vitamin B12 in the blood that cells can take up; some laboratories measure it instead of total B12.
Low B12 causes anaemia and nerve symptoms; NICE set the bands in 2024.
Levels in UK guidance
Ranges differ between labs — your report shows your lab's range.
Vitamin B12
- Above 350 ng/L total (or above 70 pmol/L active) is where NICE says deficiency is unlikely. NICE NG239 (2024), table 1
- 180–350 ng/L total (25–70 pmol/L active) is the indeterminate band: without symptoms NICE advises watching rather than treating. NICE NG239 1.3.12, 1.3.15
- Below 180 ng/L total (below 25 pmol/L active) is confirmed deficiency for the GP. NICE NG239, table 1
- Supplements taken shortly before the test can make total B12 look normal; the combined pill can lower it. NICE NG239 1.3.8
Active B12 (holotranscobalamin)
- Above 70 pmol/L is where NICE says deficiency is unlikely. NICE NG239 (2024), table 1
- 25–70 pmol/L is the indeterminate band: without symptoms NICE advises watching rather than treating. NICE NG239 1.3.12, 1.3.15
- Below 25 pmol/L is confirmed deficiency for the GP. NICE NG239, table 1
- B12 tablets, injections or patches can raise active B12 without fully treating a deficiency; in pregnancy NICE uses active B12 as the first test. NICE NG239 1.3.8, 1.3.2
What can shift the number
Now or in the last 3 months
- Pregnant. In pregnancy the usual ranges for many tests are different: show these results to your midwife or GP. NICE NG201 Antenatal care (2021); NICE NG3 Diabetes in pregnancy (2015, updated 2020); RCOG Green-top Guideline 76: thyroid disorders in pregnancy (2025; PMID 40240075); NICE NG239 (2024) rec 1.3.2; BSG guidelines on the management of abnormal liver blood tests (2018); Lab Tests Online UK: Lipid profile; C-reactive protein (CRP); ESR
Long-term conditions
- Coeliac disease, Crohn's or colitis, a stomach ulcer, or stomach or bowel surgery. Stomach or bowel surgery and some bowel conditions reduce B12 absorption. NICE NG239 Vitamin B12 deficiency in over 16s (2024), box 2; MHRA Drug Safety Update: Metformin and reduced vitamin B12 levels (2022)
Some medicines and supplements can change this result too. When you add your results, Hea asks which ones you take.
What a GP reads it with
Vitamin B12
Read together with: Folate, Haemoglobin, MCV (mean cell volume). NICE NG239 Vitamin B12 deficiency in over 16s (2024), box 1 and rec 1.2.3; BSH guidelines for cobalamin and folate disorders (2014; PMID 24942828)
A GP usually asks about symptoms and diet, may add methylmalonic acid or folate, and treats with tablets or injections depending on the cause.
Active B12 (holotranscobalamin)
Read together with: Folate, Haemoglobin, MCV (mean cell volume). NICE NG239 Vitamin B12 deficiency in over 16s (2024), box 1 and rec 1.2.3; BSH guidelines for cobalamin and folate disorders (2014; PMID 24942828)
A GP usually asks about symptoms and diet, may add methylmalonic acid or folate, and treats with tablets or injections depending on the cause.
Questions to ask your GP
- Do my symptoms fit a B12 problem?
- Was I taking a supplement before this test?
- Should folate be checked as well?
Safety first
If your lab or GP has flagged your result as urgent, follow their advice first.
Not for emergencies: call 999 or NHS 111.
Questions people ask
What are the symptoms of B12 deficiency?
The NHS lists symptoms such as feeling weak or tired, shortness of breath, headaches, and a sore or red tongue with mouth ulcers. B12 deficiency can also affect the nerves, causing pins and needles, numbness, muscle weakness, and problems with balance, memory or mood. The NHS says it is important to be diagnosed and treated as soon as possible, so see a GP if you have symptoms.
NHS: Vitamin B12 or folate deficiency anaemia – symptoms, Page last reviewed 20 February 2023What causes low B12?
The NHS says the most common cause in the UK is pernicious anaemia, where the immune system attacks healthy cells in the stomach. Other causes include not getting enough B12 from your diet, some stomach or bowel conditions, stomach operations, and some medicines.
NHS: Vitamin B12 or folate deficiency anaemia – causes, Page last reviewed 20 February 2023How is B12 deficiency treated?
Treatment depends on the cause, and your GP decides it. The NHS says B12 deficiency anaemia is usually treated with injections; if it is caused by your diet, tablets may be used instead. Take any B12 treatment as your GP or pharmacist advises.
NHS: Vitamin B12 or folate deficiency anaemia – treatment, Page last reviewed 20 February 2023Which foods contain vitamin B12?
The NHS says vitamin B12 is found in meat, fish, eggs, dairy products and specially fortified foods. Some people become low in B12 because they do not get enough from their diet. See a GP if you think you may have a B12 deficiency.
NHS: Vitamin B12 or folate deficiency anaemia, Page last reviewed 20 February 2023Will I need another blood test after starting treatment?
Often, yes. The NHS says a blood test is often done around 7 to 10 days after starting treatment to check it is working. Another may be done after about 8 weeks to confirm the treatment has worked.
NHS: Vitamin B12 or folate deficiency anaemia – treatment, Page last reviewed 20 February 2023Got your result? Start with this question in your AI.
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Sources
Each line about the test above names the guideline it comes from, as Hea's guide cites it. The answers to the questions come from:
- NHS: Vitamin B12 or folate deficiency anaemia – symptoms, Page last reviewed 20 February 2023
- NHS: Vitamin B12 or folate deficiency anaemia – causes, Page last reviewed 20 February 2023
- NHS: Vitamin B12 or folate deficiency anaemia – treatment, Page last reviewed 20 February 2023
- NHS: Vitamin B12 or folate deficiency anaemia, Page last reviewed 20 February 2023