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.