Vitamin B12: About This Test

What is it?

This blood test measures the amount of vitamin B12 in your blood. Your body needs this B vitamin to make blood cells and to keep your nervous system healthy.

Why is this test done?

This test is used to:

• Check for vitamin B12 deficiency anemia. The test is often done for those who've had stomach or bowel surgery. It's also done for those who have problems of the small intestine. And people may have the test if they have a family history of this anemia.
• Check on the B12 level for someone who is on a diet that restricts certain foods (such as vegan).
• Find the cause of other types of anemia. This includes megaloblastic anemia. Folate is usually measured at the same time. Lack of either vitamin can lead to this type of anemia.
• Help find the cause of a decrease in mental abilities. It also can help find the cause of nervous system symptoms. These can include tingling or numbness of the arms or legs.
• See if a person has vitamin B12 deficiency anemia after being diagnosed with atrophic gastritis.

How do you prepare for the test?

You may need to fast for 8 hours before the test. Your doctor may give you some specific instructions.

How is the test done?

A health professional uses a needle to take a blood sample, usually from the arm.

How long does the test take?

The test will take a few minutes.

What happens after the test?

• You will probably be able to go home right away.
• You can go back to your usual activities right away.