A-315 Pregnant Women Showed Lower Incidence Rates of False-positive Treponemal Antibody Screening Results When Compared with Nonpregnant Women and Men at an Appalachian Academic Medical Center
Authors: Yang J , Tacker D 
Published: September 2023

Abstract

Background
Screening for syphilis infection is recommended for all pregnant women and any adult or adolescent with increased risk for infection. Many clinical labs have adopted the reverse syphilis serology testing algorithm, that employs an automated and high-throughput treponemal antibody test as the initial screen followed by a non-treponemal antibody test to confirm all reactive samples. For specimens with discordant results, a second treponemal antibody test is used as the adjudicator. Syphilis serological tests can have false-positive results, causing discordant results between screening test and confirmatory test and complicating result interpretation. It is controversial from the literature whether pregnancy is a cause of false-positive results. This study is to determine the incidence rate of false-positive treponemal screening results among pregnant and nonpregnant patients, and to assess whether false-positivity is associated with pregnancy and age.

Methods
This is a retrospective study of sequential results from two treponemal antibody screening tests, including BioPlex 2200 Syphilis Total immunoassay between May 2020 and Jan 2022, and Alinity i Syphilis TP assay between Feb 2022 and Nov 2022. For each screening test, the incidence rates of false-positive results were calculated based on the reverse algorithm criteria and compared among pregnant women, nonpregnant women and men. The age differences between patients with false-positive results and patients with nonreactive results were also evaluated.

Results
Overall, 0.32% of 9 575 samples tested by Alinity assay showed false-positive results not confirmed by RPR or TP-PA. This is significantly lower (P &lt; 0.01) than 0.60% of 14 983 samples observed for BioPlex assay. However, the lower incidence rate of false-positive results for Alinity assay relative to the Bioplex assay is only observed in pregnant women (0.18% vs 0.44%, P &lt; 0.01) and nonpregnant women (0.32% vs 0.71%, P = 0.06), but not in men (0.68% vs 0.63%, P = 0.81). For both assays, male patients with false-positive results had an older median age than those with nonreactive results, and female patients showed the opposite trend, although statistical significance was only observed in nonpregnant women for Bioplex assay (median [IQR], 25 [19–35] vs 30 [22–43], P = 0.047). For both assays, pregnant women had lower incidence rates of false-positive results than nonpregnant women, although the differences did not reach statistical significance without age adjustment (Bioplex assay: 0.44% vs 0.71%, P = 0.07; Alinity assay: 0.18% vs 0.32%, P = 0.26). When age is adjusted, binary logistic regression analysis showed that pregnant women had significantly lower incidence rates of false-positive results relative to nonpregnant women for both assays (BioPlex assay: P = 0.049; Alinity assay: P &lt; 0.01).

Conclusion
Although the incidence rates of false-positive results in BioPlex and Alinity treponemal antibody screening tests were overall within a low range, a significantly lower rate was observed for Alinity assay in pregnant women but not in men. For both assays, pregnant women showed a significantly lower incidence rate of false-positive results in comparison with nonpregnant women after age adjustment.