Coccidioides complement fixation

Definition

Coccidioides complement fixation is a blood test that looks for antibodies to the fungus Coccidioides immitis. The fungus causes the disease coccidioidomycosis.

How the test is performed

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 Coccidioides immitis. 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. There are several other serology techniques that can be used depending on the suspected antibodies. They include agglutination and enzyme immunoassay.

How to prepare for the test

There is no special preparation for the test.

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 used to detect infection with the fungus coccidioidomycosis, which can cause lung or widespread (disseminated) infection.

Normal Values

No coccidioides antibodies are detected.

What abnormal results mean

Abnormal results mean that coccidioides antibodies are present. This can indicate a current or prior infection.

The test may be repeated after several weeks to detect a rise in titer (antibody count), which helps to confirm an active infection.

In general, the worse the infection, the higher the titer, except in patients with weakened immune systems.

There can be false positive tests in patients with other fungal diseases such as histoplasmosis and blastomycosis, and false negative tests in patients with single lung masses from coccidioidomycosis.

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

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.

References

Pappagianis D. Serologic studies in coccidioidomycosis. Semin Respir Infect. 2001 Dec;16(4):242-50.

Illustrations

Blood test

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Coccidioides antibody test