Vitamin B12

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. Strict vegetarians (vegans) who do not eat animal products and babies of mothers who are strict vegetarians are at increased risk for developing anemia and should take a supplement containing vitamin B12. 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 a folic acid test, because a lack of either one 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 dementia 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

Do not eat or drink (other than water) for 10 to 12 hours before the test.

How It Is Done

Your health professional drawing blood will:

Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.

Clean the needle site with alcohol.

Put the needle into the vein. More than one needle stick may be needed.

Attach a tube to the needle to fill it with blood.

Remove the band from your arm when enough blood is collected.

Put a gauze pad or cotton ball over the needle site as the needle is removed.

Put pressure to the site and then a bandage.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Risks

There is very little chance of a problem from having a blood sample taken from a vein.

You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.

In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.

Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin,warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.