Vitamin B12 (cobalamin) is needed to form red blood cells, for the nervous system and for DNA synthesis. The body does not produce it — we get B12 only from animal food.
Its deficiency develops slowly and insidiously: it hits both the blood (anemia) and the nerves, and the neurological consequences can be irreversible if left too long.
Reference ranges
Approximate reference:
| Normal | about 200 – 900 pg/mL |
| Deficiency | < 200 pg/mL |
| Gray zone | 200 – 300 pg/mL — clarified with extra tests |
What an elevated level means
A high B12 in the blood is not toxic in itself (the excess is excreted). But it can sometimes be a marker of other conditions — liver disease, kidney disease, certain blood disorders — so a persistently high level without supplement use is worth discussing with a doctor.
What a low level means
B12 deficiency shows up as two groups of symptoms:
Blood: anemia — fatigue, weakness, pallor, breathlessness.
Nerves: tingling and numbness in the hands and feet, unsteady gait, worsening memory and mood. It is important not to miss these symptoms.
Who is at risk: vegans and vegetarians, the elderly (poorer absorption), people after stomach/intestinal surgery, and those taking metformin and acid-lowering drugs.
How the test is done
Venous blood. Preferably fasting; do not take B12 supplements before the test, so as not to distort the result.
With borderline values a doctor may order homocysteine and methylmalonic acid — they show tissue-level deficiency more accurately.
What affects the value
- Diet — B12 is only in animal food; vegans are at risk.
- Age — absorption in the stomach worsens over the years.
- Stomach acidity — heartburn drugs reduce uptake.
- Metformin — long-term use worsens B12 absorption.
- GI surgery — impairs uptake.
Calculate using this marker
Calculators where Vitamin B12 (cobalamin) is used directly:
Frequently asked questions
Who should definitely test B12?
Vegans and vegetarians, older adults, those on long-term metformin or heartburn drugs, and people after stomach or bowel surgery. In these groups deficiency develops more often and unnoticed.
Why is B12 deficiency dangerous?
For two reasons: anemia (fatigue, weakness) and nerve damage (numbness, tingling, unsteadiness, worsening memory). The neurological consequences of an advanced deficiency can become irreversible — so it is important not to delay.
B12 is normal but I have symptoms — what should I do?
A "normal" blood level does not always rule out tissue deficiency. In borderline cases homocysteine and methylmalonic acid are tested — they are more sensitive. Discuss this with your doctor.