Vitamins & minerals
Vitamin B12 blood test
A vitamin B12 blood test measures the amount of B12 (cobalamin) in your blood, a vitamin your body needs to make healthy red blood cells and keep nerves and DNA working. A low B12 can cause tiredness, tingling and problems with memory or mood, and it is often checked alongside folate and iron.
- What it measures
- Vitamin B12 in your blood (total serum B12)
- Sample
- Venous blood sample
- Fasting
- Not required
- Also known as
- B12, cobalamin, serum B12
- Included in 57 Health tests
- 6 panels, from £135
Vitamin B12 reference range
Guide only| Category | Guide range |
|---|---|
| Adults (total serum B12) | 200–900 ng/L |
Below about 200 ng/L usually suggests deficiency, but a “grey zone” (roughly 150–250 ng/L) can still cause symptoms, where an active B12 (holotranscobalamin) or MMA test can help. Recent B12 supplements or injections raise the result. ng/L is the same as pg/mL.
These are general UK adult guide ranges from public health sources, shown for orientation only. Reference ranges vary by laboratory, assay, age and sex, so your result is always interpreted against the range quoted by the analysing laboratory on your report.
What does a high vitamin B12 level mean?
A high B12 is usually not harmful in itself and most often reflects recent supplements or B12 injections.
Less commonly, a persistently high B12 without supplementation can be associated with liver conditions or other medical issues, so it is interpreted in context rather than treated as a result on its own.
What does a low vitamin B12 level mean?
A low B12 means your B12 stores are low and can cause tiredness, pins and needles, a sore tongue, and problems with memory, concentration or mood. Left untreated it can lead to a type of anaemia and nerve problems.
Common causes include a vegan or vegetarian diet, pernicious anaemia (an autoimmune loss of the protein needed to absorb B12), other absorption problems, and long-term use of metformin or acid-reducing medicines (PPIs).
When to consider a vitamin B12 test
- Ongoing tiredness or low energy
- Tingling, numbness or “pins and needles”
- A sore or red tongue, or mouth ulcers
- Memory, concentration or mood changes
- A vegan or vegetarian diet
- Long-term metformin or PPI (acid-reducer) use
- A personal or family history of autoimmune conditions
Frequently asked questions
This page is general information about a blood marker and is not medical advice or a diagnosis. A vitamin B12 test screens and gives context; results are interpreted by a clinician alongside your symptoms, history and other markers. Always follow the guidance on your own report.