Serologic test for B. anthracis is a blood test to look for antibodies against the bacteria that causes anthrax.

Blood is 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.

The blood sample is sent to a lab where serology tests are done to look for B. anthracis, the bacteria that causes anthrax.

There is no special preparation.

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 may be performed when anthrax is suspected.

A normal result shows no antibodies present. During the first few days to weeks of exposure to an antigen, however, there may be slight antibody production. As the disease progresses, more antibodies will be present. If a disease is suspected, the test may need to be repeated 10 days - 2 weeks after the first test.

An abnormal result means antibodies to B. anthracis have been detected and you may have anthrax. However, some people are exposed to the bacteria and do not develop the disease. It is important to see an increase in the antibody count after a few weeks in order to diagnose a current (rather than a previous) infection.

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.

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)

A serology test can determine if a patient has ever been exposed to a particular antigen, but this does not necessarily indicate a current infection. A person who tests positive for B. anthracis may not develop anthrax.

The best test for anthrax is a culture of affected tissue or blood.

Mandell GL, Bennett JE, Dolin R. Principles and Practice of Infectious Diseases. 6th ed. London: Churchill Livingstone; 2005.

McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. St. Louis, Mo: WB Saunders; 2006; 741.