Syphilis Immunoassay Signal Strength Correlates with Active Infection in Pregnant Women.

Objective This study aimed to evaluate the association of ARCHITECT chemiluminescent immunoassay (CIA) signal strength (signal-to-cutoff [S/CO] ratio), with maternal syphilis stage, rapid plasma reagin (RPR) reactivity, and congenital syphilis. Study Design A prospective observational study of reverse syphilis screening was conducted. Pregnant women were screened with CIA. Reactive CIA was reflexed to RPR; particle agglutination test ( Treponema pallidum particle agglutination [TPPA]) was performed for CIA+/RPR- results. Clinical staging with history and physical was performed, and disease stage was determined. Prior treatment was confirmed. We compared S/CO ratio and neonatal outcomes among the following groups: Group 1 : CIA+/RPR+/TPPA+ or CIA+/RPR-/TPPA+ with active syphilis; Group 2 : CIA+/RPR-/TPPA+ or CIA+/serofast RPR/TPPA+, previously treated; Group 3 : CIA+/RPR-/TPPA+, no history of treatment or active disease; Group 4 : CIA+/RPR-/TPPA-, false-positive CIA. Results A total of 144 women delivered with reactive CIA: 38 (26%) in Group 1, 69 (48%) in Group 2, 20 (14%) in Group 3, and 17 (12%) in Group 4. Mean (+/- standard deviation) S/CO ratio was 18.3 +/- 5.4, 12.1 +/- 5.3, 9.1 +/- 4.6, and 1.9 +/- 0.8, respectively ( p < 0.001). Neonates with overt congenital syphilis occurred exclusively in Group 1. Conclusion Women with active syphilis based on treatment history, clinical staging, and laboratory indices have higher CIA S/CO ratio and are more likely to deliver neonates with overt evidence of congenital syphilis.