Serological analysis of specific IgA to Chlamydia pneumoniae: increased sensitivity of IgA antibody detection using prolonged incubation and high antigen concentration.

The microimmunofluorescence technique (MIF) is recognized as the only test hitherto allowing discrimination between different Chlamydia species and is considered to be the reference method for serology. This method was developed for the detection of IgG and IgM antibodies only. We investigated the effects of some test parameters on the ability of MIF to detect Chlamydia pneumoniae IgA. These parameters were the time needed for binding of serum IgA to C. pneumoniae antigen and the effect of antigen concentration on the outcome of IgA antibody testing. It was found that the most sensitive MIF tests for the detection of serum IgA antibodies were those in which an overnight incubation of sera with antigen slides containing high concentrations of chlamydial elementary bodies was employed. The number of patients with chronic infections found to have elevated IgA titers was increased by 25% using longer incubation times for the antibody-antigen reaction. Thirty-two sera from patients with coronary artery disease and confirmed chronic C. pneumoniae infection were used to compare antigen slides with low and high concentrations of elementary bodies with respect to IgA levels; 31/32 patients were found to have specific IgA antibodies to C. pneumoniae using the high antigen concentration, as opposed to only 22/32 patients using the low antigen concentration.