Evaluation of False Positive RPR Results and the Impact of SARS-CoV-2 Vaccination in a Clinical Population with a High Rate of Syphilis Utilizing the Traditional Screening Algorithm
Authors: Xiong-Hang Kang , MacLennan Anne , Love Sara , Saenger Amy 
Published: November 2022

Abstract

Background
Syphilis infection caused by the bacterium Treponema pallidum is a sexually transmitted infection leading to serious health complications if undiagnosed or untreated. Serologic diagnostic methods include a combined approach for detection of antibodies to non-treponemal (RPR) and treponemal antigens using a traditional or reverse algorithm. The BioPlex2200 Syphilis Total/RPR assay (BioRad Laboratories) is an automated method for simultaneous detection of treponemal/non-treponemal antibodies, but recently recalled due to potential RPR false reactivity following SARS-CoV-2 mRNA vaccination. The purpose of this study was to evaluate syphilis confirmation and false positive RPR rates over various intervals preceding and during the COVID-19 pandemic in a population with a high rate of syphilis, and a subset of pregnant females screened for syphilis prior to delivery, to assess the impact of mRNA vaccines on different RPR reactivity rates and methodologies.

Methods
The study cohort included all individuals screened for syphilis (n=48,288) and a subset of pregnant patients (n=2,944), where syphilis testing was ordered for routine clinical care utilizing the traditional algorithm. Five timepoints represented various phases and RPR methods utilized throughout the pandemic: 1) pre-COVID-19 (automated RPR, July – December, 2019); 2) COVID-19, pre-mRNA vaccine (manual RPR, July – December, 2020); 3) COVID-19, mRNA vaccine (manual RPR, January – June, 2021); 4) COVID-19, mRNA vaccine/boosters (automated RPR, August – December, 2021); and 5) COVID-19, mRNA vaccine/boosters (RPR/T.pallidium sent out to reference laboratory, January – February, 2022). False positive RPR was defined as reactive RPR/nonreactive T.pallidium. Titers were determined for all reactive RPR specimens.

Results
Syphilis confirmation rates were between 4.5 – 7.9% for all patients and 0.15 – 0.41% for pregnant patients during the entire study period. False positive RPR rates increased in the screening (5% to 8.6%) and pregnant groups (4.4% to 7.5%) from 2019 to 2021. Comparing only timepoint 1 (2019) to timepoint 4 (end of 2021), both reporting automated RPR, overall reactivity rates increased from 8.2% to 14.1% in the screening cohort and 4.8% to 7.8% in the pregnant cohort. No significant differences were observed in manual RPR reactivity (5.2% vs. 6.3%) or false positive rates (1.3% vs. 1.2%) pre- and post-vaccination in the screening or pregnant population (reactivity rates: 0.68% vs. 0.29%; false positives: 0.27% vs. 0.15%). Over 90% of RPR false positives had titers less than 1:4. False positive RPRs decreased from 8.6% to 2.9% following implementation of RPR sendouts.

Conclusions
RPR false reactivity rates increased with SARS-CoV-2 vaccination but is specific to the BioPlex2200 RPR assay which has improved analytical sensitivity for anti-lipoidal antibodies compared to manual RPR methods, thus may be more susceptible to false positives post-vaccination. Increased RPR reactivity rates were clinically notable likely because the traditional syphilis screening algorithm is utilized at our institution.