Vitamin B12

The lower limit varies widely between laboratory methods (from about 130 to 200 pmol/L) and many laboratories print ng/L rather than pmol/L; it falls in pregnancy and on some medicines; use the range your lab printed.

Vitamin B12 is needed to make red blood cells and to keep nerves healthy. It comes from animal foods and is absorbed in the last part of the small intestine with the help of a protein made in the stomach called intrinsic factor. The test measures the total amount in the blood, most of which is bound to a carrier that does not deliver it to cells, which is why it is an imperfect guide.

Doctors request it when the red cells are large, the haemoglobin is low, or someone has tingling, numbness, balance problems, memory change or tiredness. It is also checked in people on a vegan diet, after stomach or bowel surgery, and on long-term metformin or acid-suppressing medicines.

• B12 injections or high-strength supplements, for weeks to months
• Liver disease
• Some blood conditions, including some leukaemias
• Long-term kidney disease
• Some cancers

• A diet with little animal food, including vegan diets
• Pernicious anaemia, an autoimmune condition that stops absorption
• Stomach or bowel surgery and conditions of the small intestine such as coeliac and Crohn's disease
• Metformin and acid-suppressing medicines, over years
• Older age
• Pregnancy, because the carrier proteins change
• Nitrous oxide exposure

• It falls through pregnancy and on the combined pill because of changes in carrier proteins, without any shortage in the cells
• A value in the lower part of the range is common in the general population and does not on its own mean a shortage; active B12 or methylmalonic acid help to sort this out
• A recent injection or supplement raises it for a long time
• Different laboratory methods give different numbers, and some are affected by intrinsic factor antibodies

No fasting needed, though some laboratories prefer it. Best taken before any supplements are started. Read with folate and a full blood count.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.