Evaluation of verification assays in EIA specimens presumptively positive for Chlamydia trachomatis

Verification of specimens positive for Chlamydia trachomatis by enzyme immunoassay (EIA) has been recommended when testing low prevalence populations. This study compared direct fluorescent antibody (DFA) and blocking antibody (BLA) verification assays in specimens presumptively positive for C. trachomatis by the Syva Microtrak II EIA. Of 1785 specimens originally tested by EIA, 96 were presumptively positive for C. trachomatis. Verification assays were concordant in 86 specimens (69 positive, 17 negative); nine of the remaining samples gave positive results in a second EIA and one was unresolved. Both verification assays gave some false-negative results. When initial EIA absorbance values were correlated with verified results, all EIA false positive results had absorbances in the low range (less than a three-fold increase over assay cut-off values). Verification of EIA results by both DFA and BLA was effective in detecting false positive results, but confining verification to low-value positive specimens could be considered for cost effective C. trachomatis testing.