Vitamin B12 (cobalamin) blood test: levels, causes and treatment

The test measures the concentration of vitamin B12 in the serum, the liquid part of blood. Lab Tests Online UK explains that B12 and folate are both B vitamins needed for normal red cell formation, tissue repair and DNA synthesis, and that B12 is also important for nerve health. B12 comes from animal products such as red meat, fish, poultry, milk and eggs, and increasingly from fortified foods.

The body stores several years' worth of B12 in the liver, which is why dietary deficiency is slow to develop and why the symptoms often creep up gradually.

It is mainly requested when a full blood count shows large red blood cells, to help diagnose the cause of macrocytic anaemia, and when symptoms suggest deficiency. The NHS lists extreme tiredness, lack of energy, pins and needles, a sore and red tongue, mouth ulcers, muscle weakness, vision problems, psychological problems from mild depression or anxiety to confusion, and problems with memory and judgement.

Lab Tests Online UK adds dizziness, weakness, fatigue, numbness or abnormal sensations in the toes or fingertips, clumsiness, and mental or behavioural changes, especially in older adults. B12 can also be monitored to check treatment, particularly in people who cannot absorb it and need lifelong treatment.

No fasting is needed for a standard B12 test. Blood is taken from a vein in the arm. Certain medicines and pregnancy may affect the result, and your healthcare professional will advise which to stop, if any. Mention any B12 injections or supplements you take.

Lab Tests Online UK describes three routes: insufficient intake, malabsorption and increased need. Pernicious anaemia, an autoimmune condition in which antibodies attack stomach cells or intrinsic factor so that B12 cannot be absorbed in the small bowel, is the most common cause. Surgery that removes part of the stomach or intestines, Crohn's disease affecting the ileum, bacterial overgrowth, reduced stomach acid, alcohol misuse and the type 2 diabetes drug metformin can all lower B12.

The NHS says dietary lack is uncommon but can happen on a vegan diet without supplements or fortified foods, that anticonvulsants and proton pump inhibitors can affect absorption, and that deficiency affects around 1 in 10 people aged 75 or over and 1 in 20 aged 65 to 74.

A symptomatic person with a low B12 is likely to have some degree of deficiency, but the number does not reflect the severity of anaemia or nerve damage. Lab Tests Online UK stresses that it is possible to have low B12 or folate levels without any clinical problems, partly because the standard serum test does not directly measure whether there is a genuine shortage of active B12 in the tissues.

When the result is borderline and symptoms are present, further tests such as methylmalonic acid, homocysteine, active B12 or intrinsic factor antibodies may help. Treatment is sometimes started on symptoms and the clinical picture, as the NHS says deficiencies can often be diagnosed from symptoms and a blood test.

The NHS says most B12 and folate deficiency can be treated with injections or tablets. B12 is usually given by injection at first, then depending on whether the cause relates to diet, either tablets between meals or regular injections; treatment may last until levels improve or for life. Many symptoms improve with treatment but some problems, particularly nerve damage, can be permanent, which is why prompt treatment matters.

Folate is usually checked at the same time: the NHS notes that a lack of either vitamin causes abnormally large red blood cells, so the two deficiencies can look alike on a blood count.