Radioimmunoassay: a sensitive screening test for histoplasmosis and blastomycosis

Currently available serologic tests for histoplasmosis and blastomycosis have limited sensitivity. In order to develop a more sensitive screening test, the radioimmunoassay (RIA) technique was adapted to detect circulating antibodies to Histoplasma mycelial, Histoplasma yeast, and Blastomyces yeast antigens. Results were compared with those of complement fixation (CF) using serums from 4 groups of subjects. Group 1 (n = 104) consisted of consecutive healthy blood donors from an endemic area. Group 2 (n = 8) had active histoplasmosis and Group 3 (n = 12) had active blastomycosis. Group 4 (n = 30) had other chronic lung diseases. Among Group 1 subjects, elevated titers to at least 1 antigen were found in 13% using RIA and 21% using CF. Only 50% of patients with active mycoses (Groups 2 and 3) had elevated CF titers to a homologous antigen, whereas the RIA test was positive in 19 of 20 patients (95%). Elevated titers to 1 or more antigens was found in Groups 1 and 4, and frequent cross-reactions among the antigens occurred with both tests, indicating low specificities for both RIA and CF. The RIA is a sensitive and rapid screening test for histoplasmosis and blastomycosis, and a negative RIA using the 3 antigens tested makes the diagnosis of active infection very unlikely in immunologically competent patients.