Evaluation of enzyme-linked immunosorbent assay for Chlamydophila pneumoniae-specific immunoglobulin M in acute respiratory tract infection

Background and objective: The study evaluated a newly developed ELISA (Hitazyme Chlamydophila pneumoniae) for detecting anti‐C. pneumoniae‐specific IgM antibody, by comparing the ELISA assay to a microimmunofluorescence (MIF) test and immunoblotting. Methods: One hundred patients with acute respiratory tract infections (58 children and 42 adults) were enrolled in the study. Paired sera were obtained from all subjects for serological testing of C. pneumoniae. Results: C. pneumoniae IgM positivity was observed in 36 (62.0%) children and 11 (26.1%) adults. However, MIF test or immunoblot revealed only four positive reactions in these patients. These four IgM‐positive patients were also positive by ELISA. A significant increase in IgG and/or IgA antibody titres in paired sera was observed in three of the four patients. Of the remaining 96 patients, no significant increase in IgG or IgA antibody titre in the paired sera was observed. To confirm the positive reactivity of ELISA, positive sera were also analysed by recombinant enzyme immunoassay. Forty‐three cases that were IgM‐positive only by ELISA were all negative by recombinant enzyme immunoassay and the ELISA results were considered to be false‐positives. Conclusions: These results indicate that a newly developed ELISA for detecting anti‐C. pneumoniae‐specific IgM antibody frequently generates false‐positive findings in patients with acute respiratory tract infections, at the current cut‐off level. Further studies are needed to determine an appropriate cut‐off level and the possible causes of the false‐positive results in the ELISA.