Abstract

Microimmunofluorescence (MIF) serology is commonly used in the diagnosis of chlamydial infections. In the MIF assay, Chlamydia pneumoniae elementary bodies were used to detect C. pneumoniae immunoglobulin G (IgG) and IgM antibodies in paired serum samples from 286 patients with respiratory illnesses. In 69 patients, MIF serology was compared with C. pneumoniae cultures. All C. pneumoniae cultures remained negative. However, 205 (71%) of 286 patients were C. pneumoniae antibody positive and 64 (22%) had MIF test results indicating recent infection; 11 showed a fourfold increase in IgG titer, 18 had IgG titers of greater than or equal to 1:512, and 41 had IgM titers of greater than or equal to 1:16. In 35 (55%) of 64 patients, a recent-infection diagnosis was based on C. pneumoniae IgM antibodies only. However, 78% of C. pneumoniae IgM-positive patients had circulating rheumatoid factor (RF) by rheumatoid arthritis latex assay. RF positivity increased with age. After absorption with anti-human IgG, all C. pneumoniae IgM-positive sera became C. pneumoniae IgM negative in the MIF assay. Twenty-five patients with active rheumatoid arthritis but without respiratory illness were also tested; 14 were C. pneumoniae IgG positive and C. pneumoniae IgM positive as well. Absorption of IgG from these RF-containing sera invariably resulted in disappearance of reactivity in the MIF IgM assay. We conclude that with age the serologic diagnosis of recent C. pneumoniae infection becomes increasingly prone to false-positive results unless sera are routinely absorbed prior to MIF IgM testing.