Evaluation of the Bio–Rad Syphilis IgG test performed on the CODA system for serologic diagnosis of syphilis

The performance of the Bio–Rad Syphilis IgG EIA test as a “screen for syphilis” [testing first by EIA and then by the rapid plasma reagin (RPR) assay if the EIA was positive or equivocal] and as a confirmatory test was evaluated by comparing results to those obtained by CAPTIA Syphilis-G. Discrepancies were resolved by repeating both EIAs and/or the SeroDia TP-PA (a particle agglutination assay that replaced the microhemagglutination Treponema pallidum test). Both EIAs were totally automated, the Bio–Rad test using the AutoPrep instrument for pipetting and the CODA system to perform all of the steps required to complete the EIA and interpret results, and the CAPTIA test using the LabOTech® to accomplish both functions. Of 449 unselected sera submitted to “screen for syphilis,” both EIAs agreed for 432 (96.2%) specimens: 395 negative, 36 positive, and one equivocal. Fifty-four specimens were positive or equivocal by one or both EIAs; 41 of these were RPR reactive. Three of these 41 were incorrectly called negative by Bio–Rad (sensitivity 92.7%), and there was 1 false-negative result by CAPTIA (sensitivity, 97.6%) ( P, not significant). To further evaluate the Bio–Rad assay as a confirmatory test, 144 known RPR-reactive specimens were tested by both EIAs. Results agreed for 134 (93.1%): 123 positive, 11 negative. After resolving discrepancies, there were 3 false-negative and no false-positive results by Bio–Rad (sensitivity 97.8%, specificity 100%), and with CAPTIA there were no false-negative results and 1 false-positive (sensitivity 100%, specificity 91.7%) ( P, not significant). The sensitivity of the Bio–Rad assay could be improved, without altering specificity, by lowering the cut-off value for equivocal results. In summary, the Bio–Rad Syphilis IgG EIA performed using the AutoPrep instrument and CODA system is a reliable, efficient method of syphilis testing.