Seasonal Increase in Syphilis Positive Testing Rate in Whole Blood Donors 2018-2021

ABSTRACT

Background/Case Studies On December 17th 2021 the U.S Food and Drug Administration published a letter to clinical laboratory staff and health care providers detailing a risk of false Rapid Plasma Reagin (RPR) when using the Bio-Rad Laboratories BioPlex 2200 Syphilis Total & RPR kit in people who had received a COVID-19 vaccination. Specifically, this notice stated that Treponema pallidum particle agglutination (TP-PA) assays did not appear to be impacted by this issue. At our institution it has been observed that since 2018, the positivity rate of syphilis screening with negative confirmatory testing has been dramatically increased from previous years. Curiously a striking number of these positives occurred at the end of the year, mainly October through December. Study Design/Methods: All whole blood (WB) donations from 2011-2021 which demonstrated positive syphilis screening with negative confirmatory testing were evaluated. Screening for syphilis was performed using the Beckman Coulter PK TP Microhemagglutination assay with confirmatory testing using CAPTIA Syphilis (T. pallidum)-G. Results/Findings: There were 77 whole blood donations from 59 unique donors screened positive for syphilis with negative confirmatory testing from 2011-2021 (summarized in table 1). A dramatic increase in the unconfirmed syphilis positivity rate was observed in 2018-2021 (mean 0.439%) compared to 2011-2017 (mean 0.024%, unpaired t-test p-value 0.0010), representing an 18-fold increase in positive screens. Of these 77 donations, 8 donors contributed 26 units (median 3, range 2-5) between 2018-2021. Three of these 8 donors made several (6, 31, and 85) WB donations with negative syphilis screening prior to becoming positive. The 5 remaining donors switched back and forth between negative and positive over the course their donation history. Conclusion(s) There has been a statistically significant increase in unconfirmed syphilis positivity rate among whole blood donors at our institute since 2018 when compared to the 7 years prior. Additionally, the positivity rate doubled from 2020 to 2021. No changes were made to the testing assay used during this time period that could explain these results. There appears to be an autumnal peak in unconfirmed positives suggesting a possible environmental trigger such as viral infection or influenza/COVID-19 (for the 2021 increase) vaccination. Further investigation would be needed to confirm such a hypothesis. (Table Presented).