The incidence of infection was determined in patients with chronic obstructive pulmonary diseases (COPD) by prospective serial serology. Chlamydia-specific IgG, IgM and IgA antibodies were detected with a recombinant DNA lipopolysaccharide (LPS) ELISA as well as with a micro-immunofluorescence (MIF) assay with elementary bodies. From 271 consecutive COPD patients who visited the outpatient clinic of the department of pulmonary diseases (211 males, 60 females, age range 34-88 years, mean age 66 SD 10 years), blood samples (n = 1058) were taken every 2-7 months; the observation period ranged from 3 to 19 months (mean 15 SD 4). The prevalence of chlamydial IgG was 72% with the MIF and 53% with the rDNA LPS ELISA. More than 90% of the COPD patients had no significant changes in their chlamydia-specific IgG, IgA and IgM titres in either test during the observation period. Seven (3%) patients had MIF results indicating acute infection during their surveillance period, of whom five were confirmed by rDNA LPS ELISA. Eleven (4%) additional patients were infected during observation, as determined by rDNA LPS ELISA only. These patients had significantly elevated -specific IgG and IgA MIF titres, as compared with the patients without infection. All 18 patients with serological evidence of acute infection during their surveillance period were re-tested in a commercial MIF test that can distinguish between , and LPS-specific antibodies, but no evidence of or infection was found. The incidence of chlamydial infection was 2.2 and 5.3/100 person-years, when diagnosed by MIF and rDNA LPS ELISA, respectively. It is concluded that the rDNA LPS chlamydia assay may currently be the most sensitive serological tool for diagnosing recent respiratory chlamydia infections and that infection occurs frequently in COPD patients.


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