Vitamin B12 test: what the result means

Vitamin B12 is needed for DNA synthesis, for red blood cells to mature and for the protective sheath around nerve fibres. The body does not make it — we get it from meat, fish, eggs, dairy products or supplements, and absorbing it requires intrinsic factor, which the stomach produces. That is why a deficiency arises not only from diet, but also from stomach or bowel disease, after stomach surgery, and when taking acid-reducing medicines or metformin. The stores held in the liver last several years, so a deficiency develops slowly. Its signs are megaloblastic anaemia (large red blood cells, a raised MCV) and tiredness, as well as nervous-system problems: numbness, tingling, weakness, balance or memory problems, which can appear even before the anaemia. A more precise marker of early deficiency is active B12 (holotranscobalamin). B12 is often tested together with folate and a complete blood count.

• When a complete blood count finds anaemia with a raised red cell volume (MCV) — megaloblastic anaemia
• With nervous-system signs: numbness, tingling, weakness, balance problems, peripheral neuropathy
• With confusion, worsening memory or signs of dementia — especially at an older age
• On a plant-based diet, after stomach or bowel surgery, with coeliac disease or Crohn's disease
• When metformin or medicines that reduce stomach acid are taken long term

A low B12 means a possible deficiency. The causes are too little animal food, absorption problems (coeliac disease, Crohn's disease, a lack of intrinsic factor due to autoimmune gastritis or after stomach surgery), an overactive thyroid, pregnancy and some medicines. A deficiency develops slowly and can damage nerves even before anaemia appears, so it matters to recognise it early.

• Too little B12 from food — a plant-based diet without supplements
• Absorption problems — coeliac disease, Crohn's disease
• A lack of intrinsic factor — autoimmune gastritis (pernicious anaemia), the state after stomach surgery
• An overactive thyroid
• Pregnancy
• Medicines: metformin, proton pump inhibitors, H2 blockers, colchicine, phenytoin

Possible symptoms are tiredness and weakness, numbness or tingling in the hands and feet, balance problems, worsening memory and confusion, a burning tongue and pale skin. What is usually assessed next: a complete blood count and MCV — whether anaemia is already present; folate, because a lack of it gives a similar blood picture; the cause — diet, the state of the stomach and bowel, medicines — since that determines whether food is enough, or whether supplements are needed and in what form; and any nervous-system signs, because they decide how urgent it is.

A high B12 is uncommon and usually means that supplements or injections are being taken. If no supplements are involved, a raised level may be linked to liver disease (cirrhosis, hepatitis) or to disorders of blood production such as polycythaemia vera or chronic myeloid leukaemia — a result like this is assessed by a doctor.

• B12 supplements or injections
• Liver disease — cirrhosis, hepatitis
• Myeloproliferative disorders — polycythaemia vera, chronic myeloid leukaemia

What is usually checked next: whether you take supplements or injections — if you do, a raised level is expected; and liver markers together with a complete blood count if you do not.

At the Medicina practica laboratory, a vitamin B12 test costs from €27 (September 2026 pricing). When there is anaemia or nervous-system signs, a family doctor may order the test free of charge.