Iron, vitamins and folate
Vitamin B12 blood test: what a high or low result means
What a vitamin B12 blood test measures, what low or high B12 can mean, and what to ask your doctor.
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Explain my results → Available on web, iOS & Android.What is vitamin B12?
Vitamin B12 helps your body make healthy red blood cells and keeps your nerves working properly. It comes from animal foods such as meat, fish, eggs and dairy, and from fortified foods. A vitamin B12 blood test checks whether you have enough. Some labs also offer an active B12 test, which measures the part your body can use.
Why is this test done?
- To find the cause of anaemia, especially when red cells are larger than usual (a high MCV)
- To investigate tiredness, pins and needles, numbness, memory problems or a sore tongue
- To check people at higher risk, such as vegans, older people or people taking metformin
- To monitor B12 treatment
What does a high vitamin B12 mean?
A high B12 result is less common and usually not harmful in itself.
Possible reasons include:
- Taking B12 supplements or having B12 injections
- Liver conditions
- Kidney conditions
- Less commonly, some blood conditions
What does a low vitamin B12 mean?
Low B12 can cause anaemia and, if it continues, problems with the nerves, so it is important to find and treat the cause. Nerve symptoms such as numbness, unsteadiness or memory problems should be assessed promptly.
Possible reasons include:
- Pernicious anaemia, an autoimmune condition that stops B12 being absorbed
- A vegan diet without supplements or fortified foods
- Stomach or bowel surgery, or conditions such as coeliac or Crohn's disease
- Long-term use of some medicines, such as metformin or acid-reducing medicines
- Older age
- Recreational use of nitrous oxide (laughing gas)
Why we don't list a "normal range". Reference ranges differ between laboratories and can depend on your age, sex and whether you are pregnant. The range printed next to your result on your own report is the one that applies to you, so use that, and talk to your doctor about what your result means for you.
What can affect your vitamin B12 result?
- B12 supplements or injections
- Pregnancy, which can lower the result
- Some medicines, such as metformin and the contraceptive pill
- Results near the bottom of the range, which can be hard to interpret and may need further tests
Make sense of the whole report, not just one line
Results are easier to understand together. Patiently AI explains your report as a whole and writes personalised questions for your next appointment.
Try it with your own results → Paste, upload or photograph your results.Questions to ask your doctor
- Is my B12 low enough to explain my symptoms?
- What might be causing it, and do I need a test for pernicious anaemia?
- Do I need tablets or injections, and for how long?
- Could any of my medicines be lowering my B12?
Frequently asked questions
What are the symptoms of low vitamin B12?
Common symptoms include tiredness, breathlessness, pins and needles, numbness, a sore red tongue, mouth ulcers and problems with memory or mood. Some people have no symptoms.
Why might I need B12 injections rather than tablets?
If your body cannot absorb B12 from your gut, for example in pernicious anaemia, tablets may not work well, so injections deliver B12 directly. If the cause is diet, tablets are often enough.
Does a normal B12 result rule out deficiency?
Not always. Results near the lower end of the range can be borderline. If your symptoms suggest deficiency, your doctor may arrange further tests.
Can Patiently AI explain my vitamin B12 result?
Yes. Paste your results, or upload a photo, PDF or Word file, and Patiently AI explains them in plain English, sticking to the numbers and ranges on your own report rather than adding its own, and suggests questions to ask your doctor. It is free, needs no account, and removes identifying details on your device first.