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.
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.
A vitamin B12 test measures the amount of vitamin B12 in the blood.
The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
- 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 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 his or her food.
- Low levels are linked with hyperthyroidism or folic acid deficiency anemia.
- High levels of protein in the blood, such as from multiple myeloma, can falsely decrease blood vitamin B12 levels.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Taking certain medicines that might affect your test results. Tell your doctor all of the medicines that you take.
- Being pregnant or breast-feeding.
- Taking large doses of vitamin C.
- Drinking large amounts of alcohol.
- Having a test, such as a computed tomography (CT) scan, that used dyes in the past 7 days.
- Having pernicious anemia, which means you lack the substance (intrinsic factor) needed to absorb vitamin B12.
What To Think About
- Folic acid levels can be high in people who lack vitamin B12. A folic acid test is often done at the same time as a vitamin B12 test. For more information, see the topic Folic Acid.
- Methylmalonic acid is a substance in the blood that increases when vitamin B12 levels decrease. A methylmalonic acid (MMA) blood test may be done to help evaluate vitamin B12 test results.
- A change in homocysteine levels may also affect a change in vitamin B12 levels because of metabolism changes. For more information, see the topic Homocysteine.
Other Works Consulted
Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.
|Primary Medical Reviewer||E. Gregory Thompson, MD - Internal Medicine|
|Specialist Medical Reviewer||Brian Leber, MDCM, FRCPC - Hematology|
|Last Revised||December 14, 2010|