Confirmatory assay increases specificity of the chlamydiazyme test for Chlamydia trachomatis infection of the cervix.

Enzyme immunoassays for the detection of chlamydial antigens are commonly used to diagnose Chlamydia trachomatis infection. As is true for all nonculture methods, the specificities of these tests are a concern. A confirmatory blocking assay (Abbott Laboratories, North Chicago, Ill.) was evaluated at four sexually transmitted disease test sites. This assay is designed to confirm true-positive Chlamydiazyme (CZ) specimens and to identify false-positive CZ reactions caused by cross-reacting bacteria. Cervical specimens were collected from 2,891 women. Chlamydia prevalence by tissue culture (TC) was 9.2% (266 of 2,891 specimens). Compared with TC, the sensitivity and specificity of CZ were 78.9% (210 of 266 specimens) and 98.2% (2,577 of 2,625 specimens), respectively. There were 48 CZ false-positive reactions. The direct fluorescent-antibody test (DFA) was positive for 31 of 48 false-positive reactions, indicating culture misses. Thus, when the standard was both TC and DFA, CZ sensitivity was 81.1% and CZ specificity was 99.3%. Of the 17 CZ-positive patients who were negative by both TC and DFA, 3 were negative on repeat CZ and 11 of 14 were identified as false positive by the confirmatory assay. The confirmatory test was positive for CZ-positive women who were positive by TC or DFA. Use of the confirmatory test, which increased the specificity to 99.9%, would increase confidence in positive CZ results and make the test more useful for screening populations with a low prevalence of C. trachomatis infection.