A radioimmunoassay (RIA) was designed and compared with
complement fixation and immunodiffusion tests for their relative ability to detect
antibodies in sera of
histoplasmosis patients. M
antigen, purified from
histoplasmin, was fixed to microtiter wells as the solid phase, and specific rabbit 125I-labeled anti-M
globulin was the source of
indicator antibodies. The optimal concentrations for the competitive-binding assay were 1.6 ng per well for M
antigen and 650 ng per well for the 125I-labeled anti-M
globulin. A panel of sera from 29
histoplasmosis patients and from patients with other
mycoses was screened for RIA activity and in
complement fixation and immunodiffusion tests that used
histoplasmin and Histoplasma capsulatum yeast-form
antigens. The sera of 22
histoplasmosis patients reacted in the RIA, 21 in the
complement fixation, and 16 in the immunodiffusion tests. Sera of patients with other mycotic
infections did not react in the RIA, with the exception of those of one
blastomycosis patient and one
candidiasis patient. The RIA could be modified to quantitate M
antigen; as little as 125 pg could be detected. The evaluation of this panel of
histoplasmosis patients' sera showed that the RIA was about equivalent in sensitivity to the
complement fixation test. Some advantages of the RIA over the
complement fixation test were that RIA was less prone to cross-reactions and gave better quantitation of low-titered sera. The RIA was a 1-day test, was not hindered by the anti-complementary activity of some sera, and could be modified to quantitate minute amounts of M
antigen.