Serologic test for B. anthracis

Alternative names
Anthrax serology test; Antibody test for anthrax

Definition
This is a blood test to detect the presence of antibodies against B. anthracis.

How the test is performed

Blood is drawn from a vein 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 distend (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 tourniquet 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.

The blood is then analyzed in a laboratory. Serology refers to the study of serum for its Antibody content. The B. anthracis microorganism stimulates the body to produce antibodies during an active infection. In the laboratory, the antibodies react with antigens in specific ways.

There are several serology techniques that can be used depending on the suspected antibodies. Serology techniques include agglutination, precipitation, complement-fixation, fluorescent antibodies, and others.

How to prepare for the test
There is no special preparation.

For infants and children:
The preparation you can provide for this test depends on your child’s age and experience. For specific information regarding how you can prepare your child, see the following topics:

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 may be performed when Anthrax is suspected.

Normal Values

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 to 2 weeks after the first test.

What abnormal results mean


If antibodies are detected, there has been exposure to B. anthracis (possible Anthrax). Some people are exposed and do not develop the disease. It is important to see an increase in the antibody titer (count) after a few weeks in order to diagnose a current (rather than a previous) infection.

Treatment with antibiotics can sometimes reduce the antibody response, making titers lower than they would have been without treatment.

What the risks are


The risks associated with having blood drawn are:

     
  • 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

Special considerations

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.

If repeated tests show markedly increased response, current infection is more likely. In this case, actual symptoms will most likely dictate the best course of therapy before any changes in laboratory test values.

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.

Johns Hopkins patient information

Last revised: December 4, 2012
by Harutyun Medina, M.D.

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