Diagnosis of coccidioidomycosis by antigen detection using cross-reaction with a Histoplasma antigen.

Background: In 2005, patients with coccidioidomycosis were observed to have positive Histoplasma antigen test results.
Methods: We performed a review of the records of patients with coccidioidomycosis who were under our care who underwent testing for Histoplasma antigen to determine the value of this test in the diagnosis of coccidioidomycosis. Many of the patients were immunosuppressed and critically ill.
Results: The Histoplasma antigen test had positive results when urine samples from 11 (58%) of 19 patients who had acute or chronic coccidioidomycosis were tested. The sensitivity was highest for patients who had acute coccidioidomycosis, and antigenuria was detected in 11 (79%) of 14 patients. One patient who had chronic coccidioidomycosis but who had a negative result when a urine sample was tested had antigen detected in bronchoalveolar lavage fluid.
Conclusions: Physicians should be alerted that infections with Coccidioides species may cause positive Histoplasma antigen test results. There is potential for the use of this test in the diagnosis of coccidioidomycosis by taking advantage of this observed cross-reactivity. The greatest benefit appears to be in the population of seriously ill patients with acute pneumonia who live in areas that are endemic for Coccidioides infection.