A vitamin B12 test measures the amount of vitamin B12 in the blood. The body needs this B vitamin to make blood cells and to maintain a healthy nervous system.
Vitamin B12 is found in animal products such as meat, shellfish, milk, cheese, and eggs. Most people who eat animal products are not likely to develop vitamin B12 deficiency anemia unless their bodies can't absorb it from food. People who don’t eat any animal products, such as vegans, need to take a vitamin B12 supplement. If you don’t eat any animal products and you are breastfeeding, your baby can be at risk for vitamin B12 deficiency, so your baby will also need to take a B12 supplement.
Vitamin B12 is stored in the liver for a year or more, which reduces a person's risk of anemia.
Vitamin B12 is usually measured at the same time as folate, because a lack of either one or both can lead to a form of anemia called megaloblastic anemia. Lack of vitamin B12 also affects the nervous system.
Why It Is Done
A vitamin B12 test is used to:
- Check for vitamin B12 deficiency anemia. There are several risk factors for this anemia, such as those who have had stomach or intestinal surgery, small intestine problems, or people with a family history of this anemia.
- Diagnose the cause of certain types of anemia, such as megaloblastic anemia.
- Help find the cause of a decrease in mental abilities or other nervous system symptoms, such as tingling or numbness of the arms or legs (peripheral neuropathy).
- See if vitamin B12 deficiency anemia is present after a person has been diagnosed with atrophic gastritis.
How To Prepare
You may need to fast for 8 hours before the test. Your doctor may give you some specific instructions.
How It Is Done
A health professional uses a needle to take a blood sample, usually from the arm.
How long the test takes
The test will take a few minutes.
How It Feels
When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.
Risks
There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.
Results
Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.
High values
- High levels of vitamin B12 can occur in liver disease (such as cirrhosis or hepatitis) and some types of leukemia. But the vitamin B12 test is not usually used to diagnose these problems.
- In rare cases, high levels may be found in people with diabetes or who are obese.
Low values
- Low levels of vitamin B12 may mean you have vitamin B12 deficiency anemia, which might be caused by problems with the absorption of the vitamin (such as pernicious anemia).
- Low levels may also occur following removal of part or all of the stomach (gastrectomy), gastric bypass surgery, or gastric stapling surgery, or following surgery to remove part of the small intestine where this vitamin is absorbed (terminal ileum).
- Low levels may mean an infection with a parasite called fish tapeworm is present.
- In rare cases, low levels may mean a person is not getting enough vitamin B12 in their food.
- Low levels are linked with hyperthyroidism or folate deficiency anemia.
- High levels of protein in the blood, such as from multiple myeloma, can falsely decrease blood vitamin B12 levels.
Credits
Current as of: October 6, 2025
Current as of: October 6, 2025
