Abstract

IgG antibody titres against Chlamydia trachomatis were measured using a whole inclusion assay (Electro-Nucleonics Laboratories, Bethesda, Maryland, USA) in single serum samples of patients who presented for termination of pregnancy with evidence of infection with this organism (case group) and of 30 women with no evidence of infection (control group). Case and control patients were matched for age, race, marital status, history of sexually transmitted diseases, number of lifetime sexual partners, and whether they had had a new sexual partner in the preceding month. Fifty eight of 60 (97%) serum samples tested had chlamydial antibodies present at a titre of 1/8 or more. There was no significant difference between the geometric mean titres of the 30 case patients and the 28 controls with chlamydial antibodies (1/161.3 and 1/77.4 respectively). Significantly more case patients than controls, however, had a titre of 1/128 or more (p less than 0.01). The sensitivity, specificity, and positive and negative predictive values of a single serum antibody titre of 1/128 or more as an indicator of recent infection compared with isolation of the organism in cycloheximide treated McCoy cells or antigen detection by Chlamydiazyme (Abbot Laboratories; North Chicago, Illinois, USA), or both, were each 67%. The value of the antibody assay seems to be questionable as high levels of antibodies failed to correlate with isolation or antigen detection results, or both, in our population.