Vitamin B12
Cobalamin
Vitamin B12 is needed for red blood cell formation and nerve function. Serum B12 is the usual first test, but it is a less reliable marker than its widespread use suggests.
Reference range
pg/mL- Women
- 197–1210 pg/mL
- Men
- 197–1210 pg/mL
Reference ranges differ between laboratories and assays. Always read your result against the range printed on your own report.
What Vitamin B12 measures
B12 is required for making red blood cells and for maintaining the myelin sheath around nerves. The body cannot produce it; it comes from animal-derived foods or supplements.
Absorption is unusually elaborate. B12 has to be released from food by stomach acid, bind to a protein called intrinsic factor made by the stomach lining, and be taken up in the final section of the small intestine. A problem at any of those steps causes deficiency regardless of intake.
The liver stores enough for several years, which is why deficiency develops slowly and is often advanced before it is noticed. Neurological symptoms can appear before any change shows in the blood count.
Why it gets tested
To investigate fatigue, tingling or numbness in hands and feet, memory or concentration problems, or an anaemia with enlarged red cells. Also monitored in people on a vegan diet, on metformin or long-term acid suppression, and after gastric surgery.
Commonly associated with low values
- Insufficient intake, most commonly on a vegan or strongly plant-based diet
- Pernicious anaemia, an autoimmune loss of intrinsic factor
- Long-term proton pump inhibitors or metformin
- Conditions affecting the terminal ileum, such as Crohn's disease
Commonly associated with high values
- Supplementation, by a wide margin the usual explanation
- Rarely, liver disease or a myeloproliferative disorder
What can shift the result
- Serum B12 measures total circulating B12, not the fraction the body can actually use — deficiency can exist within the normal range
- Where the result is borderline, methylmalonic acid or holotranscobalamin is more informative
- Any supplement taken shortly before the test inflates the value
Questions
Can B12 be deficient despite a normal result?
Yes. Serum B12 includes a large fraction bound to a protein that cannot deliver it to cells, so the total can look adequate while the usable amount is not. Where symptoms suggest deficiency and the serum value is borderline, methylmalonic acid or holotranscobalamin gives a clearer answer.
Who is most at risk of B12 deficiency?
People on vegan or strongly plant-based diets, anyone taking metformin or acid-suppressing medication long term, people who have had stomach or small bowel surgery, and older adults, in whom absorption declines. Risk depends at least as much on absorption as on intake.
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This page is general information about a laboratory value. It is not a diagnosis and does not replace medical advice. Discuss your own results with a doctor, who can read them in the context of your history and symptoms.