Background: The reverse sequence algorithm (RSA) is increasingly used for prenatal syphilis screening by high volume laboratories, beginning with a treponemal test such as the chemiluminescene immunoassay (CIA), followed by reflex testing of CIA-reactive specimens with the rapid plasma regain test (RPR). Implications of discordant serology (CIA-reactive and RPR-non-reactive) for pregnancy and neonatal outcomes are unknown.
Methods: Pregnant women at Kaiser Permanente Northern California with CIA-reactive, RPR-non-reactive serology underwent reflex testing with Treponema pallidum particle agglutination (TPPA) from August 2007-August 2010. Samples reactive with TPPA (CIA+, RPR-, TPPA+) were deemed “TPPA-confirmed”; those reactive only with CIA (CIA+, RPR-, TPPA-) were deemed “unconfirmed CIA-reactive”. Past syphilis testing history, other medical history, maternal and neonatal outcomes and treatment were abstracted from the medical record.
Results: Of 194 pregnant women with discordant treponemal serology, 156 (80%) were unconfirmed CIA-reactive. Of 106 women who were retested, 43 (41%) became CIA-non-reactive; all were initially unconfirmed CIA-reactive. Women with TP-PA-confirmed serology were more likely to receive treatment for syphilis than those with unconfirmed CIA-reactive serology (31.5% vs 3.8% p<0.01). Of 3 women who seroconverted to CIA-reactive and RPR-reactive during pregnancy, 1 became persistently CIA-non-reactive later in pregnancy and may have had both a false positive RPR and CIA. A large majority of pregnancies (189, 97.5%) ended in a live birth; there were no stillbirths attributable to syphilis. No infants were born with clinical signs of congenital syphilis, and 2 infants received antibiotic therapy based on the mother’s serology results.
Conclusions: Most pregnant women with discordant treponemal serology were unconfirmed CIA-reactive and a substantial percentage became CIA-non-reactive upon repeat testing. Isolated CIA-reactive serology in these populations is likely to be falsely positive. Repeated testing of unconfirmed CIA-reactive specimens and reflex testing of discordant specimens with a second treponemal test is crucial in this population.