Inclusion fluorescent-antibody test as a screening assay for detection of antibodies to Chlamydia pneumoniae.

A study was conducted to determine the ability of the inclusion immunofluorescence assay (inclusion IFA) to act as a screening test to detect samples with antibodies to Chlamydia pneumoniae; microimmunofluorescence (MIF) was used as the "gold standard." In addition, the inclusion IFA was compared using HEp-2 cells infected with either C. pneumoniae CM-1 or Chlamydia trachomatis serovar E. A total of 331 serum samples representing a range of MIF titers were evaluated. The sensitivities of the inclusion IFA for detecting samples with C. pneumoniae MIF titers of > or = 16 were 96.9 and 74.8% with C. pneumoniae- and C. trachomatis-infected cells, respectively. For samples with an elevated C. pneumoniae MIF titer of > or = 512, the sensitivities of the C. pneumoniae- and C. trachomatis-based inclusion IFA were 97.0 and 8.8%, respectively. These results suggest that the inclusion IFA is not a genus-specific test, as evidenced by the failure of the C. trachomatis-infected cells to detect a significant number of samples with C. pneumoniae antibodies. Samples that had elevated C. pneumoniae inclusion IFA and MIF titers but that were found negative (titer, <16) by the C. trachomatis inclusion IFA were further tested by an in vitro neutralization assay for functional antibodies that might not have been detected by the serological assays. The in vitro neutralization results corroborated the serological results in that all seven sera tested had a neutralization titer for C. pneumoniae (range, 20 to 225), while all but one failed to have any effect on the infectivity of C. trachomatis serovar E. While the C. pneumoniae inclusion IFA had a high sensitivity for detecting chlamydial antibodies, depending on whether it was used as a screening test for detecting samples with low (> or = 16) or elevated (> or = 512) MIF titers, its specificity ranged from 53.4 to 77.1%. In conclusion, the inclusion IFA with C. pneumoniae-infected cells was best suited as a sensitive screening test for identifying specimens with elevated MIF titers (those associated with a possible acute infection with C. pneumoniae).