Delayed hypersensitivity skin tests with mycelium-derived (
coccidioidin) or spherule-derived (
spherulin)
antigens (or both) can be used to identify patients who have been sensitized to the dimorphic fungus Coccidioides immitis. Prior studies suggest that
coccidioidin and
spherulin skin test
antigens detect comparable numbers of reactors among exposed subjects. Studies in subjects residing in areas outside the United States where C. immitis is not endemic suggest that both
antigens are specific for the fungus. The specificity and reactivity of
coccidioidin and
spherulin have not been compared in nonendemic regions of the United States in which the skin test
antigens and an appropriate travel or exposure history are used to identify patients with possible C. immitis
infection. A review of delayed cutaneous reactions to
coccidioidin in 6,375 patients tested between 1970 and 1979 in the southeastern United States revealed 958 (15.0%) and 234 (5.7%) positive reactions (greater than or equal to 5 mm), respectively, at 24 and 48 h. Subsequent tests with
spherulin in 2,775 patients tested in 1980 and 1981 revealed 866 (31.2%) and 288 (10.3%) positive reactions, respectively, at 24 and 48 h. False-positive
immediate hypersensitivity reactions contributed to the large number of
spherulin reactors at 24 h. Differences among the patients sampled, work exposure, and travel history were excluded as causes of this surprising and highly significant (P less than or equal to 0.0001) difference in the 48-h delayed cutaneous reaction. These observations suggest two possibilities: (i)
spherulin is less specific than
coccidioidin, or (ii) a surprising prevalence of C. immitis sensitization exists among patients in nonendemic regions of the United States.