Retrospective comparison of false-positive result frequencies of 3 syphilis serology screening tests in pregnant and nonpregnant patients at an academic medical center in Appalachia

Abstract

Objective: This study retrospectively compared false-positive result frequencies of 3 syphilis serology screening tests and assessed whether false positivity was associated with pregnancy and age.

Methods: Results for 3 screening tests were retrieved from the laboratory database, including rapid plasma reagin (RPR) assay between October 2016 and September 2019, BioPlex 2200 Syphilis Total immunoassay between May 2020 and January 2022, and Alinity i Syphilis TP assay between February 2022 and April 2023. The false-positive result frequencies were calculated based on testing algorithm criteria.

Results: False-positive result frequency for BioPlex was 0.61% (90/14,707), significantly higher than 0.29% (50/17,447) for RPR and 0.38% (55/14,631) for Alinity (both P < .01). Patients with false-positive results were significantly older than patients with nonreactive results for RPR (median age: 36 vs 28, P < .001), but not for BioPlex or Alinity. For all 3 tests, the positive predictive values in pregnant women were lower than those in nonpregnant women or men. However, pregnant women did not exhibit a higher false-positive result frequency.

Conclusion: Although false-positive result frequencies were low overall for all 3 syphilis serology tests, there is a significant difference between different tests. Pregnancy was not associated with more false-positive results for all 3 tests.

Keywords: false positive; immunoassay; positive predictive value; pregnancy; serology test; syphilis.