The use of a confirmatory assay to increase the sensitivity and specificity of the Chlamydiazyme test

The blocking assay was used to reexamine 15,662 patient specimens (2,565 male specimens, 13,097 female specimens) that were submitted for Chlamydia detection using the Chlamydiazyme EIA assay (Abbott Laboratories, North Chicago, IL). Specimens that gave optical density (OD) readings between 1.99 to cutoff and between cutoff to 3 times the negative control in the Chlamydiazyme EIA assay were analyzed further by the blocking assay during the phase 1 study. In the phase 2 study, another 1,120 specimens (473 male specimens and 647 female specimens) that had the above mentioned OD range in the Chlamydiazyme assay were tested with the blocking assay and the direct fluorescent antibody test using the cytospin method. Significant finding from phase 1 study demonstrated that 42.3% of the male specimens with optical density between cutoff to three times the negative control can be blocked by blocking assay (confirmed positive), whereas only 50% of the female specimens with OD range between cutoff to .5 were blocked by the blocking assay. In the phase 2 study, similar results were obtained with the blocking assay. The direct fluorescent antibody test showed excellent correlation with the blocking assay. These data showed that both blocking or direct fluorescent antibody tests can be used for confirmation purposes to increase the sensitivity and specificity of the assay. However, for specimens with OD values below the cutoff to 3 times the negative control, it was necessary to reassess the specimens by either of the methods. This was true especially with the male specimens.