This is a test to determine the level of vitamin B-12 in the blood.
How the test is performed
Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.
A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
For an infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.
How to prepare for the test
Fast for 6 to 8 hours before the test. Consult with the health care provider if you are using medications that may affect test results, including colchicine, neomycin, para-aminosalicylic acid, and phenytoin.
For infants and children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics:
- infant test or procedure preparation (birth to 1 year)
- Toddler test or procedure preparation (1 to 3 years)
- Preschooler test or procedure preparation (3 to 6 years)
- School age test or procedure preparation (6 to 12 years)
- Adolescent test or procedure preparation (12 to 18 years)
How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed
This test is most often performed when other tests (which may include CBC, RBC indices, reticulocyte count, blood smear, or others) indicate the presence of a megaloblastic anemia.
Pernicious anemia is the megaloblastic anemia caused by poor absorption of vitamin B-12. This is usually caused by decreased production of intrinsic factor, a substance essential to vitamin B-12 absorption, in the stomach. This test may also be performed as part of the testing to determine the cause of nervous system disorders.
- 200-900 pg/ml (picograms per milliliter)
What abnormal results mean
Values of less than 100 pg/ml indicate a significant deficiency of vitamin B-12, with symptoms likely to be present or develop.
Causes of vitamin B-12 deficiency include:
- inadequate intake (rare except with a strict vegetarian diet)
- malabsorption diseases (for example, celiac disease and sprue)
- lack of intrinsic factor
- hypermetabolic state (for example, with hyperthyroidism)
Increased vitamin B-12 levels are uncommon, because normally excessive B-12 is excreted in the urine. However, excessive B-12 may occur with disorders that affect the levels of proteins that attach to vitamin B-12 in the blood. These disorders include:
- liver disease (such as cirrhosis or hepatitis)
- myeloproliferative disorders (for example, myelocytic leukemia)
Additional conditions under which the test may be performed:
- dementia due to metabolic causes
What the risks are
Risks associated with having blood drawn are slight:
- excessive bleeding
- fainting or feeling light-headed
- hematoma (blood accumulating under the skin)
- infection (a slight risk any time the skin is broken)
- multiple punctures to locate veins
In testing for megaloblastic anemias of any sort, serum or red blood-cell folate levels are usually also obtained.
The blood test for levels of vitamin B-12 has become much more accurate within the past few years. Now, there are fewer false-normal results, because only biologically active B-12 is measured.
The cause of a vitamin B-12 deficiency is usually determined by the Schilling test.
Hemolysis of the blood sample (rupture of some of the red blood cells) 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. Obtaining a blood sample from some people may be more difficult than from others.
by Simon D. Mitin, M.D.
All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.