A test that measure the presence and amount of antibodies in blood. These antibodies may be against a particular type of tissue, cell, bacteria, virus, or other external substance.

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, and an elastic band is placed around the upper arm to cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in air-tight vials 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.

No special preparation is necessary for this test.

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.

The antibody level in the blood is a reflection of the body's past exposure to an antigen, or to something that the body does not recognize as belonging to itself. The body uses antibodies to attack and eliminate the foreign matter.

Every living cell has different protein markers on its surface. The body's immune system identifies those cells that are not part of itself by these surface proteins.

Occasionally, the body makes an error and begins to recognize its own tissue as being non-self, or foreign. This leads to an immune response against that tissue type, cell, or substance that is part of the body. This is known as autoimmunity.

Levels of antibodies in the blood (also known as the antibody titer) are measured to determine:

• the strength of an immune response to the body's own tissue in diseases such as systemic lupus erythematosus (SLE) and other autoimmune disorders.
• the need for a booster immunization
• whether a recent vaccine administration caused a strong enough response from the immune system, bringing your antibody titer against a particular disease up to a preventive level.

In some situations your health care provider may check your antibody titer to see if you had a natural infection in the past (e.g., chickenpox) or to decide which immunizations you need.

This depends on the antibody being tested. If your health care provider is testing for antibodies against your own tissue, then the normal value would be zero or negative.

If your health care provider is testing to see if your immunization brought your antibody titer up to a preventive level, then that would be a given value determined by that immunization.

If your health care provider is testing for antibodies against your own tissue, abnormal results would show a positive antibody titer. This means that your immune system is fighting its own tissue, cells, or substances.

If your health care provider is testing to see if your immunization brought your antibody titer up to a preventive level, an abnormal result would indicate that your body has not mounted an adequate response against the immunization, and you are not adequately protected from the disease.

Risks associated with venipuncture 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

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.