Biomarker Glossary

Vitamin B12

Vitamin B12 (cobalamin) is essential for red blood cell production, nerve function and DNA synthesis. A vitamin B12 blood test is the standard UK screen for B12 deficiency, a common cause of fatigue, anaemia and neurological symptoms, particularly in older adults, vegans and people with absorption problems.

Also known as: B12, Cobalamin, Cyanocobalamin

Vitamin B12 is most informative when read alongside related markers like Ferritin and Haemoglobin. A single number rarely tells the whole story. If your reading sits outside the typical range, share the full report with your GP or healthcare professional before drawing conclusions.

What is Vitamin B12?

Vitamin B12 is found almost exclusively in animal foods (meat, fish, eggs, dairy). Your body cannot make B12 and has limited stores, so a steady supply from diet is needed.

Absorption is complex. B12 binds to intrinsic factor in the stomach and is absorbed in the small intestine. This makes deficiency common in older adults (reduced stomach acid), people on long term acid suppressants, and those with autoimmune pernicious anaemia.

B12 supports red blood cell production, nervous system function (particularly the protective myelin sheath around nerves), and the conversion of homocysteine to methionine. Deficiency can cause megaloblastic anaemia and, more seriously, irreversible nerve damage if untreated.

Why test Vitamin B12?

To investigate symptoms of fatigue, anaemia, tingling, numbness or memory problems.
To screen vegans, vegetarians, older adults and people on metformin or long term acid suppressants.
To investigate suspected pernicious anaemia or autoimmune absorption problems.
Alongside folate and ferritin in anaemia work up.
As part of a comprehensive nutritional or biohacking panel.

To investigate symptoms of fatigue, anaemia, tingling, numbness or memory problems.

Vitamin B12 normal range (UK)

Result	What it means
Above 200 ng/L (148 pmol/L)	Normal
150 to 200 ng/L (111 to 148 pmol/L)	Borderline. Symptomatic people may need treatment.
Below 150 ng/L (111 pmol/L)	Deficient
Above 1000 ng/L	Usually reflects recent supplementation or, rarely, liver disease or blood disorders

UK NHS thresholds. Many clinicians and researchers consider levels below 400 ng/L suboptimal, particularly when neurological symptoms are present. Active B12 (holotranscobalamin) and methylmalonic acid are more sensitive markers when results are borderline.

About these ranges. The ranges above are typical UK clinical lab ranges aligned to NHS and Royal College of Pathologists guidance. Your Chxhealth report will show the specific reference range used by our partner lab, Randox, for each marker. Lab ranges vary slightly between providers and assays. Always interpret your results in the context of the range printed on your own report.

What causes high Vitamin B12?

B12 supplementation (oral or injection)
Liver disease
Some blood disorders (myeloproliferative diseases)
Chronic kidney disease

What causes low Vitamin B12?

Vegan or strict vegetarian diet without supplementation
Pernicious anaemia (autoimmune destruction of stomach cells that produce intrinsic factor)
Atrophic gastritis (common in older adults, reduces stomach acid and intrinsic factor)
Long term proton pump inhibitor or H2 blocker use
Metformin (lowers B12 over years of use)
Coeliac disease, Crohn's disease and other absorption disorders
Gastric bypass surgery
Heavy alcohol use
Some genetic variants affecting B12 transport

Symptoms of high Vitamin B12

Usually no symptoms.
Mostly seen after supplementation or injection.

Symptoms of low Vitamin B12

Fatigue and weakness
Pale or yellow tinged skin
Breathlessness
Tingling or numbness in hands or feet
Difficulty walking or balance problems in severe cases
Sore, smooth or red tongue
Mouth ulcers
Mood changes, depression
Memory problems, confusion
Vision changes (rare)

How is Vitamin B12 tested?

Vitamin B12 is measured from a small blood sample. No fasting is required.

Hold any B12 supplements on the morning of the test for the most accurate baseline.

If serum B12 is borderline, ask about active B12 (holotranscobalamin) or methylmalonic acid for confirmation.

Best tested alongside folate, ferritin and a full blood count when investigating anaemia.

Chxhealth samples are analysed by Randox, a UK laboratory accredited by UKAS. Results return in 3 to 5 working days.

How to support healthy Vitamin B12 levels

Eat B12 rich foods regularly: meat, fish (particularly salmon, sardines, trout), eggs, dairy and fortified foods.
If you are vegan or vegetarian, take a B12 supplement (oral or sublingual). Cyanocobalamin and methylcobalamin are both effective.
If you are over 50, consider a daily B12 supplement even if you eat animal foods, as absorption naturally declines with age.
If you take metformin or long term acid suppressants, check B12 annually.
Address coeliac disease or other absorption problems where present.
For confirmed pernicious anaemia, B12 injections (usually every 3 months) are the standard NHS treatment.

These are general lifestyle suggestions. Chxhealth is an information service. For personal medical advice, please speak to a healthcare professional.

Chxhealth panels that test Vitamin B12

12 Chxhealth biomarker panels include Vitamin B12. Each is analysed by Randox, a UK laboratory accredited by UKAS. Reports are delivered in 3 to 5 working days.

Vitamin B12 FAQs

What is a good vitamin B12 level?

Above 200 ng/L is the NHS threshold for adequate, but many clinicians prefer levels above 400 ng/L for optimal nervous system function. Below 150 ng/L is considered deficient and needs treatment.

Who is at risk of B12 deficiency?

Vegans and strict vegetarians, older adults (atrophic gastritis), people on metformin, long term proton pump inhibitor users, those with coeliac or Crohn's disease, and people after gastric bypass surgery.

Can B12 deficiency be reversed?

Yes, if caught early. Oral or injected B12 reliably corrects deficiency in most people. Some neurological damage from prolonged deficiency may not fully recover, which is why early testing matters.

Is B12 from supplements as good as B12 from food?

Yes. Cyanocobalamin and methylcobalamin are both well absorbed in supplement form, and are the recommended option for vegans and anyone with confirmed deficiency.

Why does metformin lower B12?

Metformin interferes with B12 absorption in the small intestine. Up to 30 percent of long term metformin users develop B12 deficiency. Annual testing is recommended.

Often tested with Vitamin B12

The biomarkers below are commonly investigated alongside Vitamin B12 because they reveal connected aspects of the same physiological picture:

Ferritin: Both nutrient markers commonly low in fatigue.
Haemoglobin: B12 deficiency causes macrocytic anaemia visible on FBC.
Vitamin D: Often paired in nutrient panels.

Choose the right test for your goal

If you are reading about Vitamin B12 because of a specific health goal, our buying guides walk you through which Chxhealth panels fit:

Sources and further reading

This page is informed by guidance from the NHS, NICE, Royal College of Pathologists and other UK authoritative bodies. For deeper detail or to verify the information, see:

About this page. Last reviewed: 13 May 2026. Next scheduled review: May 2027. This page has not yet been independently reviewed by a clinician. It is written from authoritative UK medical guidance (NHS, NICE, Royal College of Pathologists, peer-reviewed sources) but has not undergone formal clinical sign off.

Important. Chxhealth is a UK information service. We do not diagnose, treat or prescribe. The reference ranges and information on this page are general educational content and should not be used as a substitute for advice from a qualified healthcare professional. For any concerns about your health or results, please speak to your GP or another healthcare professional.