Serology of Chlamydia trachomatis in infants.

Sera from 502 infants with pneumonia were tested for antibodies to Chlamydia trachomatis by the microimmunofluorescence test; 175 (34.9%) were positive for IgM antibodies (titer, greater than or equal to 1:32). Chlamydiae were recovered from 42 (46.2%) of 91 IgM antibody-positive infants as compared with six (3.3%) of 181 IgM antibody-negative infants (P less than 0.0001). Two (4%) of 46 of the infants with inclusion conjunctivitis, but not pneumonia, had titers of IgM antibody of greater than or equal to 1:32; both shed the organism from the rectum. IgM antibody to C. trachomatis is not maternally transmitted to infants and was detected at a low rate (1.1%) in infants with nonpneumonic conditions. Diagnosis of pneumonia due to Chlamydia in infants by isolation of the agent is slow and unreliable. High levels of IgM antibody (greater than 1:32) appear to reflect a systemic chlamydial infection and offer the possibility of a same-day diagnosis. Thus, the detection of specific IgM antibodies to C. trachomatis may be the method of choice in diagnosing chlamydial pneumonia in infants.