This test is used to detect the presence of antibodies to Coxiella burnetii (C. burnetti) bacteria in the blood. Thehighly infectious bacteria causes Q fever.

An antibody defends the body against bacteria, viruses, fungus, or other foreign body (antigen). Certain cells tell the body to produce antibodies during an active infection.

In the initial stage of an illness, few antibodies may be detected. Antibody production increases during the course of an infection. For this reason, such tests are often repeated several weeks after the first test is done.

This complement fixation test looks to see if  the body has produced antibodies to a specific antigen -- in this case the C.burnetti bacteria. If the antibodies are present, they stick, or "fix" themselves, to the antigen, that's why the test is called "fixation."

The test specifically looks for the antibodies in the clear liquid portion of the blood called the serum. (The general term for such a technique is called serology.)

Antibodies are rarely produced in people with acute C. burnetti infections. Instead, the presence of such antibodies usually indicate chronic Q fever.

No special preparation is necessary for this test.

A needle is inserted to draw blood. You may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.

The test is performed to detect Q fever.

The presence of no antibody to C. burnetii is normal.

An abnormal result means you have a current infection with C. burnetti, or that you have been exposed to the bacteria in the past. People with past exposure may have antibodies, even if they are not aware that they were exposed. Further testing may be needed to distinguish between a current and 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)