For the serological diagnosis of acute
Chlamydia pneumoniae infection, the microimmunofluorescence (MIF) test is the most commonly used method and also the "gold standard" for the measurement of
immunoglobulin G (
IgG) and
IgM antibodies. The role of
IgA antibodies in diagnosis has not been established. Commercially available
fluorescein-labeled anti-human
IgA conjugates have not been systematically compared to each other, and this situation may cause considerable variations in
IgA results. Therefore, we tested 261 serum samples from 122 patients with
pneumonia for
IgA antibodies by using six alpha-chain-specific
anti-IgA conjugates in our in-house MIF test, one commercial MIF test, and one
enzyme immunoassay (EIA). Interfering
IgG antibodies were removed with Gullsorb
reagent before the measurement of
IgA antibodies. Altogether, 14 significant
IgA antibody increases in serum samples between the acute phase and the convalescent phase were detected by at least one of the conjugates in the MIF test, while no increases were found in the
IgA EIA. Only one patient showed a significant
IgA antibody increase with all of the
fluorescein-labeled conjugates. Five significant titer changes were detected by at least two conjugates, and in nine instances, the titer increase was detected by one conjugate only. The titer agreement indicated by kappa coefficients was very good or good for all of the
fluorescein-labeled conjugates and the EIA with low antibody titers but decreased with increasing titers.