Seasonal increase in syphilis screening reactivity rates in whole blood donors, United States, 2011–2023

Abstract

Background:

In December 2021, the U.S. Food and Drug Administration published a letter to clinical laboratory staff and healthcare 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 COVID-19 vaccination; Treponema pallidum particle agglutination assays did not appear to be impacted by this issue. We evaluated reactivity rates of syphilis screening with negative confirmatory testing at our institution by year and seasonality.

Methods:

We performed a retrospective study of routine syphilis testing of whole blood (WB) collections at an academic hospital-based donor center in the eastern United States. All WB donations from 2011 to 2023 which demonstrated reactive syphilis screening (Beckman Coulter PK TP Microhemagglutination) with negative confirmatory testing (CAPTIA Syphilis (T. pallidum)-G) were evaluated. Reactivity rates by year and season of donation were compared using unpaired t-tests.

Results:

A total of 109 WB donations from 86 unique donors who donated from 2011 to 2023 screened reactive for syphilis with negative confirmatory testing. The unconfirmed syphilis reactivity rate increased from 2018 to 2023 (mean: 0.360%) compared to 2011–2017 (mean: 0.071%, p < .05). An autumnal peak in unconfirmed reactives was observed.

Conclusion:

The unconfirmed syphilis reactivity rate among WB donors at our institution increased markedly since 2017 compared to the 7 years prior and doubled from 2020 to 2021. No testing assay changes explain these results. The autumnal peak in unconfirmed reactives suggests a possible environmental trigger such as viral infection or vaccination.

1 |. INTRODUCTION

The U.S. Food and Drug Administration (FDA) mandates blood donor testing for syphilis.^1,2 During the COVID-19 pandemic, several blood collection organizations (BCOs) in the United States and Canada noted unconfirmed increased syphilis screening test reactivity rates,³ including reports of unconfirmed repeat reactive tests for syphilis.⁴ On December 17, 2021, the FDA published a letter to clinical laboratory staff and healthcare 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 COVID-19 vaccination. When our institution investigated this issue in our syphilis testing records, we saw no change to nontreponemal testing results but incidentally observed increased test reactivity in our Beckman Coulter PK TP Microhemagglutination assay for the detection of Treponema pallidum antibodies in 2020 and 2021. Thus, we decided to explore the issue with this different assay. The objective of this study was to calculate the reactivity rate of syphilis screening with negative confirmatory testing from 2011 to 2023.

2 |. STUDY DESIGN AND METHODS

2.1 |. Institutional review board/human subjects research

Informed consent for blood donation was obtained for all whole blood (WB) donors in accordance with FDA guidelines. Blood donors provided written informed consent for the use of their blood for research purposes. This study was exempt from institutional review board approval (secondary data analysis).

2.2 |. Data source

Our data source was routine transfusion-transmitted infectious diseases testing records for all blood donors from the National Institutes of Health (NIH) Blood Bank, a hospital-based donor center located at NIH Clinical Center. Data were collected from a retrospective review into blood donor testing records compiled in an electronic software database (Blood Bank Computer Systems (BBCS) Inc., Bonney Lake, WA).

2.3 |. Inclusion and exclusion criteria

Allogeneic donors met all the eligibility criteria for volunteer WB donation, including evaluation for transfusion-transmitted infections-related risks such as travel, prescription drug use, and sexual contacts on the FDA Blood Donor History Questionnaire (DHQ). Donors who were not excluded for WB donation based on this initial routine screening were included in this study. All WB donations from 2011 to 2023 which demonstrated reactive syphilis screening with negative confirmatory testing were evaluated.

2.4 |. Donor testing

Screening for syphilis was performed at a centralized blood donor testing laboratory using the Beckman Coulter PK TP Microhemagglutination assay for the detection of T. pallidum antibodies (Beckman Coulter, Brea, California).⁵ Confirmatory testing was performed using CAPTIA Syphilis (T. pallidum)-G (Trinity Biotech, Jamestown, New York), an enzyme immunoassay for the qualitative detection of IgG antibodies to T. pallidum in serum specimens, used in conjunction with non-treponemal testing (the ASI RPR Card Test) to provide serological evidence of infection with T. pallidum and exclude a history of syphilis.⁶

2.5 |. Statistical analysis

We retrospectively reviewed all syphilis testing in our donors (i.e., the longitudinal changes in all volunteer allogeneic WB donations from January 1, 2011 to December 31, 2023 through routine blood donor screening for transfusion-transmitted diseases including syphilis). We calculated the total number of WB donations from donors who screened reactive for syphilis with negative confirmatory testing from 2011 to 2023 and the total number of associated unique donors. For each unit, we recorded a screening test status: positive, negative, and false positive (negative to unconfirmed reactive). The unconfirmed syphilis reactivity rate was defined as the total number of WB donations that had unconfirmed reactive testing for syphilis (numerator) divided by the total number of whole blood donations for that year (denominator). The mean unconfirmed syphilis reactivity rate for each year in the study period was calculated, then averaged for the time periods 2011–2017 and 2018–2023 respectively due to an observed sudden increase of unconfirmed reactive screening reactivity of unknown origin in the year 2018 compared to the previous 7 years. The mean reactivity rates for the low and high periods were compared with an unpaired t-test. Results were considered statistically significant at the 95% confidence level (p < .05). All calculations were performed in Microsoft Excel.

3 |. RESULTS

3.1 |. Data source

A total of 60,375 volunteer allogeneic WB donations from January 1, 2011 to December 31, 2023 were recorded. NIH Blood Bank collected an average of 4644 WB donations per year during the study period. The total WB donations per year fluctuated but declined over time, from a peak of 6494 donations in the year 2011 to a nadir of 3344 donations in 2020. The mean number of annual donations was 3769 donations per year in 2018–2023, compared to a mean of 5395 donations per year in 2011–2017.

3.2 |. Syphilis test reactivity rate by year

A total of 109 WB donations from 86 unique donors screened reactive for syphilis with negative confirmatory testing from 2011 to 2023. An increase in the unconfirmed syphilis mean reactivity rate was observed in 2018–2023 (0.360%) compared to 2011–2017 (0.071%, unpaired t-test p-value: .014 [p < .05]).

3.3 |. Syphilis test reactivity rate by season

Syphilis test reactivity was also analyzed by season (January–March, April–June, July–September, October–December) (Figure 1). Average reactivity rates were the highest in the fall season (October–December) relative to all other seasons in both 2011–2017 and 2018–2023 and was higher in 2018–2023 than in 2011–2017. The season with the highest number of reactive WB donations was fall 2021 (19 donations), which was almost 10 times the number of donations seen on average in the fall season earlier in the decade (mean one annual unconfirmed reactive donation each fall in 2011–2017).

FIGURE 1.

Average number of unconfirmed reactive syphilis whole blood (WB) donations and syphilis reactivity rates from 109 unique donors screened reactive for syphilis with negative confirmatory testing from the years 2011–2017 versus 2018–2023 by season of donation, NIH Blood Bank, 2011–2023.

3.4 |. First-time versus repeat donors

Eleven of our 86 unique donors (12.8%) were repeat donors. All other donors were first-time donors. The repeat donors provided a median of seven donations (range 2–31 donations) and three unconfirmed reactive donations (range 2–6 unconfirmed reactive donations) during 2011–2023.

4 |. DISCUSSION

There has been a statistically significant increase in the unconfirmed syphilis reactivity among whole blood donors at our institute since 2018 compared to reactivity rates in the 7 years prior (2011–2017). The reactivity rate doubled from 0.389% in 2020 to 0.776% in 2021, with a peak reactivity observed in the autumn months (October–December). Even as yearly donations decreased over time, the unconfirmed syphilis reactivity rate increased in 2018–2023. False positivity of any infectious diseases screening among blood donors is significant, as it results in donor deferrals, quarantined products, and time-consuming donor evaluation and reentry visits that can be costly to BCOs, but it was unexpected in this case because no changes were made to the testing assay used during this time and no other test results for potential TTIs showed seasonal changes, suggesting the need for alternative explanations.

Environmental triggers were one biologically plausible phenomenon we explored. We observed an autumnal peak in the unconfirmed reactives, suggesting that seasonal variations in viral infection and infectivity or vaccinations may be inducing false positive results. Recent publications on post-viral autoimmune reactions show that viruses such as SARS-CoV-2 and vaccination for influenza or COVID-19 (mRNA) may interfere with treponemal assays,^7,8 which may not be a new phenomenon: In the early 1990s, for example, false positive viral screening tests after vaccination were noted in testing results for human T-cell lymphotrophic virus type 1 (HTLV-1) among blood donors following influenza vaccination.⁹ The relationship of these results on the syphilis testing assay with false positive viral screening tests after vaccination (influenza and/or mRNA) is unclear. We speculate that they may be secondary to some as yet unspecified cross-reactivity between the treponemal hemagglutination assay reagent that uses preserved avian erythrocytes and the egg-derived influenza vaccine. These multifactorial hypotheses are worth investigating.

The primary strength of our study is our sample. Our sample comes from our institution’s unique, dedicated donor pool, whose base includes a disproportionate number of regular donors who have donated WB for many years. Eleven of our 86 unique donors (12.8%) provided multiple units, which allowed us to compare testing results in the same donors with the same assays over several years. This study population includes all donors in our donor base who met study inclusion criteria, so is representative of the entire donor population at our institution. Though the number of overall WB donations per year dropped over the study period, the characteristics of the donor base did not change in any meaningful sense over the time period, so did not likely contribute to these results. We cannot account for the reason for the fall in donations aside from the COVID-19 pandemic.

Our study had several limitations. First, our donor demographics are unrepresentative of the populations of (1) United States blood donors and (2) persons with syphilis in the United States. Second, our donor pool is small compared to larger BCOs. We performed a comprehensive review of all our donors who tested positive for syphilis, but that complete list yielded a small sample. Thus, results and statistical inferences may not be generalizable to other populations. Third, we could not obtain donor vaccine history or participation in new vaccine trials, so we cannot definitively attribute these results to their vaccination status.

5 |. CONCLUSION

The unconfirmed syphilis reactivity rate among WB donors at our institution has increased significantly since 2018 when compared to the 7 years prior, most markedly from 2020 to 2021 at the height of the COVID-19 pandemic. Autumnal peaks in the unconfirmed syphilis reactivity rate were observed, with fall 2021 as the season with the highest number of unconfirmed positive WB donations. No changes were made to the testing assay during this time period. Other explanations for this seasonal variation such as possible environmental triggers (e.g., viral infection, vaccination) should be explored. Reporting issues with testing for blood donor screening like this one is essential for safeguarding public health, maintaining trust, safety and reliability of the blood supply. It also allows for continuous improvement of testing protocols and procedures. By addressing any shortcomings or weaknesses in the testing process, organizations can implement changes to enhance the accuracy and reliability of screening methods.

Abbreviations:

BCOs

blood collection organizations

CDC

U.S. Centers for Disease Control and Prevention

DHQ

U.S. Food and Drug Administration Blood Donor History Questionnaire

FDA

U.S. Food and Drug Administration

PA

particle agglutination

TTIs

transfusion-transmitted infections

T. pallidum (TP)

Treponema pallidum

WB

whole blood