EVALUATION OF SEROLOGICAL METHODS IN THE DIAGNOSIS OF CHLAMYDIA-PNEUMONIAE PNEUMONIA DURING AN EPIDEMIC IN FINLAND

A complement fixation (CF) test, a micro-immunofluorescence (micro-IF) test and an enzyme immunoassay (EIA) using Re-lipopolysaccharide as antigen were compared in the diagnosis of chlamydial infection in 136 mainly elderly patients hospitalized with community-acquired pneumonia during a Chlamydia pneumoniae epidemic in Finland in 1986-1987. Chlamydial pneumonia was diagnosed in 58 (42.6 %) of the 136 pneumonia patients; 44 (75.9 %) of them could be shown by micro-IF to be caused by Chlamydia pneumoniae, three by Chlamydia psittaci and four by Chlamydia spp. Only 5 (11.4 %) of 44 patients with Chlamydia pneumoniae pneumonia were IgM-positive, indicating that the majority of cases were reinfections. In this population of mainly elderly patients the CF test was insensitive, being positive in only 6 (10.3 %) of 58 cases of chlamydial pneumonia. The EIA detected 72.4 % of cases and micro-IF 87.9 % of cases (including infections with Chlamydia pneumoniae, Chlamydia psittaci and Chlamydia spp.). In the EIA 77 % of positive cases were positive in serum samples taken a week apart, whereas the corresponding figure for micro-IF was 50 %. In micro-IF the measurement of IgA antibody levels is recommended and IgM-positive sera should be retested after removal of IgG antibody to avoid false-positive findings due to presence of rheumatoid factor. The collection of a third serum sample, for instance one month after onset, is also recommended, since half of the patients showed a diagnostic response in the micro-IF only in the sera taken one month apart.