The vitamin B12 level is a test to tell how much vitamin B12 is in your blood.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. The band is removed to let the blood flow again. Once the blood has been collected, the needle is removed. The puncture site is covered to stop any bleeding.

In infants or young children, the area is cleansed with antiseptic and punctured with a sharp needle or a small blade (lancet). The blood may be collected in a small glass tube (pipette), on a slide, onto a test strip, or into a small container. A bandage may be placed on the puncture site if there is any bleeding.

You should not eat or drink for about 6 - 8 hours before the test. Tell your health care provider if you are taking any medicines that may affect test results, including colchicine, neomycin (Neosporin), para-aminosalicylic acid, and phenytoin (Dilantin).

When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.

This test is most often done when other tests (which may include CBC, RBC indices, reticulocyte count, blood smear, or others) suggest a blood disorder called megaloblastic anemia.

Pernicious anemia is the megaloblastic anemia caused by poor vitamin B12 absorption. This can occur when the stomach makes less intrinsic factor, a substance needed for vitamin B12 absorption. This test may also be done to find the cause of nervous system problems.

Normal values are 200 - 900 pg/ml (picograms per milliliter).

Values of less than 100 pg/ml show a lack (deficiency) of vitamin B12. People with this deficiency are likely to have or develop symptoms.

Causes of vitamin B12 deficiency include:

• Not enough vitamin B12 in diet (rare except with a strict vegetarian diet)
• Poor absorption (malabsorption) diseases (for example, celiac disease and sprue)
• Lack of intrinsic factor
• Above normal heat production (hypermetabolic state - for example, with hyperthyroidism)
• Pregnancy

• Liver disease (such as cirrhosis or hepatitis)
• Myeloproliferative disorders (for example, myelocytic leukemia)

• Delirium
• Dementia
• Dementia due to metabolic causes

Risks from having blood drawn are slight:

• Excess bleeding
• Fainting or feeling light-headed
• Blood under the skin (hematoma)
• Infection
• Many needle sticks to find veins

The health care provider will usually take blood or red blood-cell folate levels when testing for megaloblastic anemias of any kind.

The blood test for levels of vitamin B12 has become much more accurate within the past few years. Now, there are fewer false-normal results, because the test only measures biologically active B12.

A Schilling test can find the cause of a vitamin B12 deficiency.

Rupture of some of the red blood cells (hemolysis) in the blood sample may affect test results.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Getting a blood sample from some people may be harder than from others.

Goldman L, Ausiello D. Cecil Textbook of Medicine. 22nd ed. Philadelphia, Pa: Saunders, 2003.

Hoffman R, Benz E, Shattil S, Furie B, Cohen H. Hematology: Basic Principles and Practice. 4th ed. Philadelphia, Pa: Churchill Livingstone, 2004.