Coccidioidin or spherulin skin test

Definition

Coccidioides is a fungus that causes coccidioidomycosis. Coccidioidomycosis is common in the San Joaquin Valley of California and certain other areas in the southwestern U.S.

Symptoms resemble pneumonia or pulmonary tuberculosis. There may be small, red, inflamed areas on the skin. This test detects exposure to this fungus.

Coccidioidin and spherulin are two different antigens and are both used to test for coccidioidomycosis. Spherulin may be a more sensitive test than coccidioidin.

How the test is performed

The test site (an area with hair, usually the forearm) is cleansed. The antigen is then injected just below the skin surface. The test is read at 24 hours and at 48 hours.

How to prepare for the test

There is no special preparation necessary.

How the test will feel

There will be a brief sting as a needle is inserted just below the skin's surface.

Why the test is performed

The test detects if the body's immune system responds to the fungus, indicating that the person has been exposed to this organism.

Normal Values

No reaction (inflammation) to the microorganism is normal.

What abnormal results mean

A positive reaction indicates there has been exposure to the microorganism. This test can be very helpful in diagnosing early infection, since the skin test can be positive 2-21 days after the onset of symptoms, even before a blood test is positive. A significant number of patients will have anergy, or will not respond to this test even when they have the disease.

There can be false-positives, in that there can be positive results when the patient has been exposed to histoplasma fungus or blastomyces fungus, but not coccidioides. A positive result may also reflect exposure to coccidioides in the distant past, rather than current infection.

What the risks are

There is a slight risk of anaphylactic shock (a severe reaction).

Special considerations

Not applicable.

Illustrations

Positive reaction to allergen