Total Vitamin B12 Blood Test

• Total vitamin B12 (measured): serum cobalamin, reported in ng/L or pmol/L depending on the laboratory.

In blood, most B12 is attached to a carrier protein called haptocorrin, which cells cannot use directly. A smaller share is attached to transcobalamin. This complex, holotranscobalamin, is often called active B12 because it is the form delivered to cells. A total B12 test measures both fractions together, so it tells you how much B12 is circulating but not exactly how much is available to tissues.

This test does not include active B12, folate, a full blood count, MMA, homocysteine or intrinsic factor antibodies. A clinician investigating possible deficiency may request some of these.

Vitamin B12 (cobalamin) comes almost entirely from animal foods such as meat, fish, eggs and dairy, and from fortified foods and supplements. Absorbing it is a multi-step process. Stomach acid releases B12 from food, a protein called intrinsic factor, made by the stomach lining, binds it, and the complex is absorbed at the far end of the small intestine. A problem at any of these points can lead to deficiency even when the diet contains plenty of B12.

Inside cells, B12 is a cofactor for two enzymes. One recycles homocysteine to methionine and supports DNA synthesis, which is why deficiency produces large, immature red cells (megaloblastic anaemia). The other handles methylmalonic acid, and its failure is thought to contribute to nerve damage. These pathways explain why homocysteine and methylmalonic acid (MMA) rise when cells are short of B12 and can be used as second-line tests.

The liver holds a large reserve, so deficiency usually develops slowly. The NHS notes that the most common cause in the UK is pernicious anaemia, now often called autoimmune gastritis, in which the immune system damages the cells that make intrinsic factor.

NICE guideline NG239 (2024) groups total B12 results as follows, while allowing laboratories to use their own method-specific thresholds:

• Below 180 ng/L (133 pmol/L): consistent with deficiency.
• 180 to 350 ng/L (133 to 258 pmol/L): indeterminate, meaning deficiency is possible and symptoms and risk factors decide what happens next.
• Above 350 ng/L (258 pmol/L): deficiency unlikely.

Several things can make a total B12 result misleading:

• Lower without true deficiency: pregnancy and oestrogen-containing contraceptives can lower total B12 without meaning cells are short of it.
• Raised by supplements: tablets, sprays and fortified drinks raise the level, which can hide an underlying absorption problem.
• Falsely reassuring: the British Society for Haematology has noted that antibodies to intrinsic factor can interfere with some assays and give a normal result in people with pernicious anaemia. Symptoms matter more than a single number.
• Unexpectedly high: a high level without supplements can occur with liver conditions and some blood disorders, and is worth discussing with your GP.

NICE advises that treatment should not be delayed while waiting for results in people with neurological symptoms or anaemia. If you have numbness, pins and needles, unsteadiness, vision changes or memory problems, see your GP promptly whatever your result. Where a result needs medical attention, we will advise you to see your GP and, with your consent, can write to them.

NICE NG239 lists a range of risk factors for B12 deficiency, including:

• a vegan diet or a diet low in animal foods, without fortified foods or supplements
• a family history of B12 deficiency or autoimmune conditions, or conditions such as atrophic gastritis, coeliac disease, type 1 diabetes or Sjögren's syndrome
• medicines including metformin, proton pump inhibitors, H2-receptor blockers, colchicine and some anticonvulsants
• gastrectomy, bariatric surgery, removal of the end of the small bowel, or abdominal or pelvic radiotherapy
• recreational nitrous oxide use

Symptoms of deficiency, such as tiredness, a sore tongue, pins and needles and low mood, overlap with many other conditions, which is one reason a blood test is useful when the picture is unclear.

May find it useful: people with one or more of the risk factors above who have not been tested recently, or who have mild, non-urgent symptoms and want a result to take to their GP.

A different test may suit you better:

• If you are pregnant: NICE recommends active B12 in pregnancy, because total B12 naturally falls. Speak to your midwife or GP.
• If you have used nitrous oxide recreationally: NICE prefers MMA or homocysteine, because B12 levels can look normal. Seek medical advice, particularly if you have numbness or weakness.
• If you already have B12 injections: routine monitoring with total B12 is not standard practice.
• If you have neurological symptoms: see your GP promptly rather than relying on a self-requested test.

Established: conditions that impair absorption, such as atrophic gastritis, become more common with age, and older adults are more likely to take long-term medicines linked with lower B12. Deficiency is a recognised and treatable cause of anaemia and nerve problems, and the NHS notes that some nervous system damage can become permanent if treatment is delayed.

Associations: the NHS lists confusion and memory problems among the possible effects of deficiency. Low B12 status has been studied as one of many factors linked with cognitive decline, but in people without deficiency an association does not show that B12 is the cause.

Not established: taking B12 when your levels are already adequate has not been shown to slow ageing or protect memory. The aim of testing is to identify and treat deficiency, not to push a normal result higher.

• Fasting is not required.
• B12 supplements, multivitamins, B12 sprays and fortified energy drinks raise the result. Tell us what you take and how recently. Do not stop or delay any prescribed B12 treatment.
• Let us know if you are pregnant or use an oestrogen-containing contraceptive or HRT, as these can lower total B12.
• B12 is commonly measured by immunoassay. If you take high-dose biotin (found in some hair, skin and nail supplements), mention it when booking so the laboratory can advise whether it could affect the result.

Choose how your sample is taken when you book. We confirm the collection option, any collection fee and the expected turnaround time before you book.

• Juvenology Clinic, Maidstone (82 King Street, Maidstone ME14 1BH): Available.
• Partner clinic near you, anywhere in the UK: Available on request. We confirm availability for your postcode before you book.
• Postal home finger-prick kit: Ask before ordering. We offer a home kit only where the laboratory confirms this test is suitable for a finger-prick sample.

Sample: Blood sample (serum for venous collection; capillary suitability confirmed by the laboratory)

Collection notes: No fasting needed. Record any B12-containing supplements and when you last took them. A postal finger-prick kit is offered only where the laboratory confirms this test is suitable for capillary samples.

The price of this test covers laboratory analysis and the laboratory's results report. The report lists each result beside the laboratory's reference range.

Beyond the blood test: optional support, priced separately

• Personalised written interpretation report: £50
• General consultation: £50
• Results consultation (results discussion and follow-up advice): £100
• Personalised programme: from £150, after we assess whether a programme suits you

Consultations are with Marina, an NMC-registered nurse. She explains what your results show and how they relate to lifestyle. No prescription service is offered with these tests.

If a result suggests you need medical follow-up, we will recommend that you see your GP and, with your consent, can provide a letter to take with you. A blood test cannot rule out every condition. If you have symptoms, speak to your GP or NHS 111, and call 999 in an emergency.

What is the difference between total and active B12?

Total B12 measures all circulating B12. Active B12 measures only the part bound to transcobalamin, which cells can take up. NICE accepts either as a first test; active B12 is preferred in pregnancy.

Can I be deficient with a normal total B12?

It is uncommon but possible, for example with assay interference or after recent supplements. If symptoms persist, your GP can consider further tests such as MMA.

My result is in the indeterminate range. What now?

NICE treats this as possible deficiency. Whether treatment or further testing is needed depends on symptoms and risk factors, so discuss it with your GP.

Does this test check for pernicious anaemia?

No. It measures B12 only. Pernicious anaemia (autoimmune gastritis) is investigated through your GP, usually with intrinsic factor antibody testing.

I take metformin or omeprazole. Should I stop?

No. These medicines are linked with lower B12, but stopping them could be harmful. Share your result with the prescriber.

Is interpretation included?

The price covers the laboratory analysis and report. A personalised written interpretation report (£50) or a results consultation (£100) can be added if you would like help understanding your result.

1. Vitamin B12 deficiency in over 16s: diagnosis and management (NG239). National Institute for Health and Care Excellence, 2024. https://www.nice.org.uk/guidance/ng239
2. Devalia V, Hamilton MS, Molloy AM; British Committee for Standards in Haematology. Guidelines for the diagnosis and treatment of cobalamin and folate disorders. British Journal of Haematology, 2014;166(4):496-513. https://pubmed.ncbi.nlm.nih.gov/24942828/
3. Green R. Vitamin B12 deficiency from the perspective of a practicing hematologist. Blood, 2017;129(19):2603-2611. https://doi.org/10.1182/blood-2016-10-569186
4. Vitamin B12 and folate. Lab Tests Online UK. https://labtestsonline.org.uk/tests/vitamin-b12-and-folate
5. Vitamin B12 or folate deficiency anaemia. NHS. https://www.nhs.uk/conditions/vitamin-b12-or-folate-deficiency-anaemia/