Association between Chemiluminescent Microparticle Immunoassay Signal-to-cutoff Ratio and Active Stage of Syphilis in Thai Pregnant Women

Abstract

Objectives: To demonstrate the association between chemiluminescent microparticle immunoassay (CMIA) signal-to-cutoff (S/CO) ratio and active stage of syphilis as well as adverse perinatal outcomes. Materials and Methods: A retrospective chart review was conducted in pregnant women with reactive CMIA (CMIA S/CO ratio ≥ 1) as the primary test in the reverse algorithm of syphilis screening. The participants were categorized into three groups: Group 1 CMIA+ venereal disease research laboratory (VDRL)+; Group 2 CMIA+ VDRL- Treponema pallidum haemagglutination test (TPHA)+; and Group 3 CMIA+ VDRL- TPHA-. CMIA S/CO ratio and perinatal outcomes were compared. Active stage of syphilis refers to having venereal disease research laboratory (VDRL) titer ≥ 1:8. Results: Eighty-three out of 8,987 (0.92%) pregnant women who came for antenatal care at Siriraj Hospital between January 2020 and February 2021 were reactive for CMIA. Two twin gestations were excluded. The CMIA S/CO ratio was highest in group 1 (n = 39) at 23.1 ± 5.5, followed by 16.1 ± 5.2 in group 2 (n = 25) and 2.1 ± 3.2 in group 3 (n = 17), p < 0.001. Perinatal outcomes were not different among the groups, except for congenital syphilis (CS). All six neonates with CS were born to the participants in group 1 who had CMIA S/CO ratio ≥ 19.9. Most of the participants who delivered neonates with CS were diagnosed with syphilis in third trimester and had VDRL titer ≥ 1:8. Conclusion: Instances of adverse perinatal outcomes and active stage of maternal syphilis were more frequent in pregnant women with higher CMIA S/CO ratio. The use of CMIA S/CO ratio as an adjunct to clinical evaluation may provide additional benefits to the syphilis screening.