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

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

You should not eat or drink for about 6 - 8 hours before the test.

Tell your health care provider about all medicines you are taking, including over-the-counter and herbal medication. Drugs that may affect test results include colchicine, neomycin, 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 blood test tests suggest a condition called megaloblastic anemia.Pernicious anemia is a form of megaloblastic anemia caused by poor vitamin B12 absorption. This can occur when the stomach makes less of the substance the body needs to properly absorb vitamin B12.

Your doctor may also order a Vitamin B12 test if you have certain nervous system symptoms. Low levels of B12 can cause numbness or tingling in the arms and legs, weakness, and loss of balance.

• Delirium
• Dementia
• Dementia due to metabolic causes

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

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about what your specific test results mean.

Values of less than 200 pg/mL are a sign of a vitamin B12 deficiency. People with this deficiency are likely to have or develop symptoms. Older adults with vitamin B12 levels between 200 and 500 pg/mL may also have symptoms.

Causes of vitamin B12 deficiency include:

• Not enough vitamin B12 in diet (rare except with a strict vegetarian diet)
• Diseases that cause malabsorption (for example, celiac disease and Crohn's disease)
• Lack of intrinsic factor
• Above normal heat production (for example, with hyperthyroidism)
• Pregnancy

Increased vitamin B12 levels are uncommon. Usually excess vitamin B12 is removed in the urine.

Conditions that can increase B12 levels include:

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

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

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

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.

Antony AC. Megaloblastic anemias. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 170.

Reynolds E. Vitamin B12, folic acid, and the nervous system. The Lancet Neurology. Nov 2006;5(11):949-60.