Syphilis Immunoassay Signal Strength Correlates with Active Infection in Pregnant Women

Abstract

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.

Keywords

ARCHITECT Syphilis TP immunoassay - congenital syphilis - reverse sequence screening algorithm - signal-to-cutoff ratio - signal strength - pregnancy

Note

This study was presented as a poster presentation at the 39th Annual Pregnancy Meeting of the Society for Maternal-Fetal Medicine in Las Vegas, NV (February 11–16, 2019). Abstract #615.

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