Comparison of Sandwich Solid-Phase Radioimmunoassay and Two Enzyme-Linked Immunosorbent Assays for Detection of Histoplasma capsulatum Polysaccharide Antigen

Detection of a Histoplasma capsulatum polysaccharide antigen (HPA) has proved a useful approach to diagnosis of histoplasmosis. Two sandwich enzyme-linked immunosorbent assays (ELISAs) using alkaline phosphatase (AP)-or horseradish peroxidase (HRP)-conjugated antibodies were compared with solid-phase radioimmunoassay (RIA) for detection of HPA. The AP-ELISA and HRP-ELISA were each positive in 17 (89.5%) of 19 urine specimens from patients with disseminated histoplasmosis, while the RIA was positive in 18 (94.7%) of 19. Specimens from patients with nondisseminated histoplasmosis were positive by AP-ELISA in 8 of 32, by HRP-ELISA in 4 of 25, and by RIA in 12 of 25. Of control specimens from patients with other fungal infections, the AP-ELISA was negative in 22 (91.7%) of 24, the HRP-ELISA in 23 (95.8%) of 24, and the RIA in 23 (95.8%) of 24. Reproducibilities AP-ELISA and HRP-ELISA were 95.1% and 95.1%, respectively. Thus, AP-ELISA and HRP-ELISA appear less sensitive than RIA and may be falsely negative in specimens containing low levels of HPA.