Enzyme-linked immunosorbent assay for antigens of Coccidioides immitis: human sera interference corrected by acidification-heat extraction.

We describe the distortion caused by sera from patients with coccidioidomycosis in an enzyme-linked immunosorbent assay (ELISA) for coccidioidal antigen. In experimental mixtures without exogenous antigen, immune sera produced an antigen-like effect. In contrast, in mixtures with a relatively large concentration of exogenous antigen, immune sera obscured antigen activity. In both circumstances, preparing specimens with an acidification-heat extraction procedure before assay rectified serum interference, and only in specimens with no exogenous antigen was the correction incomplete. Other evidence, including direct measurement of anticoccidioidal antibody after extraction, suggested that the residual antigen-like activity was not from persistent antibody. Incorporating the extraction procedure into the ELISA of clinical specimens, we found antigen activity in nine of 15 specimens. We conclude that acidification-heat extraction of sera improves detection of coccidioidal antigens and that coccidioidal antigens are present in some patients with coccidioidal infection.