Confirmation of positive results for chlamydial antigen by the Chlamydiazyme assay: value of repeated testing and a blocking antibody assay.

We studied the specificity of the Abbott Chlamydiazyme test for detection of Chlamydia trachomatis antigen by means of a specific blocking antibody test. A total of 457 previously positive specimens were tested; 22 did not block in the blocking antibody test, 39 did not repeat as positive, and 396 were confirmed as positive. The distribution of A492 values obtained with specimens which did not repeat as positive was nonrandom and was concentrated between the cutoff values and 0.400. The positive predictive value of the Chlamydiazyme assay after initial testing was 86.7% (396 of 457), but the positive predictive value increased to 94.7% (396 of 418) if specimens which were not repeatedly positive were considered negative. We recommend routinely repeating the Chlamydiazyme assay for all specimens which give A492 values between the cutoff and 0.400 to eliminate many false-positive results. Use of the blocking antibody reagent can then be reserved for confirming only specimens which are repeatedly positive.