Research Summary

CDC Provides New Recommendations for Syphilis Testing as Cases Surge

Anthony Calabro, MA

According to the CDC, there were more than 207,000 cases of syphilis (all stages and congenital) reported in the United States in 2022, the most cases reported since 1950. The sharp rise in cases represents an increase of 17% from the previous year and 78.9% from just 2018.

“The most alarming concerns center around the syphilis and congenital syphilis epidemics, signaling an urgent need for swift innovation and collaboration from all STI prevention partners,” the CDC wrote in its 2022 STI Surveillance Report.1

In response, the CDC released its first-ever recommendations for syphilis testing in the laboratory, including guidelines on serologic testing and the methods for identifying Treponema pallidum, the bacterium that causes syphilis.

“This report details CDC’s new recommendations for syphilis testing, including laboratory-based tests, point-of-care (POC) tests, processing of samples, and reporting of test results to aid laboratorians and clinicians in the diagnosis of syphilis,” the authors wrote.2

The authors of the report noted that these recommendations are intended for clinical laboratory directors and their staff, clinicians, and disease control personnel “who must choose among the multiple available testing methods, establish standard operating procedures for collecting and processing specimens, interpret test results for laboratory reporting, and counsel and treat patients.”

The CDC made full recommendations on serologic syphilis testing, as well as endpoint titers, algorithms for serologic syphilis testing, serologic syphilis testing for pregnant persons and those living with HIV/AIDS, the direct detection of T. pallidum by darkfield microscopy, and direct detection of T. pallidum by immunohistochemistry and silver staining.

See below for the full list of recommendations.2

    CDC Laboratory Recommendations for Syphilis Testing2

    Recommendation for endpoint titers
    Endpoint titers (the highest dilution yielding a reactive result) should be determined and clearly reported when testing serum with nontreponemal (lipoidal antigen) assays that detect antibodies to lipoidal antigens (i.e., rapid plasma reagin [RPR] and venereal disease research laboratory [VDRL] tests). Reports should not contain mathematical symbols such as > or < signs.

    Recommendation for syphilis serologic testing algorithm
    Serologic tests that measure antibodies to both nontreponemal (lipoidal) and treponemal antigens related to syphilitic infections should be used in combination, when the primary test is reactive, to aid in the diagnosis of syphilis. Sole reliance on one reactive serologic test result can misclassify a patient’s syphilis status. Both the traditional syphilis screening algorithm (initial screening with nontreponemal [lipoidal antigen] assays) and the reverse syphilis screening algorithm (initial screening with treponemal immunoassays) are acceptable. The preferred algorithm should be based on laboratory resources, including staff, space and costs, test volume, and patient populations served.

    Recommendation for serologic syphilis testing
    Nontreponemal (lipoidal antigen) tests (e.g., RPR or VDRL) are not interchangeable when used to determine antibody titers; testing on follow-up samples must be performed with the same type of test. The TPPA test is the preferred manual treponemal test.

    Recommendation for syphilis serologic testing in pregnant persons
    Nontreponemal (lipoidal antigen) and treponemal tests should be interpreted in the same manner regardless of pregnancy status.

    Recommendation for syphilis serologic testing in persons living with HIV/AIDS
    Nontreponemal (lipoidal antigen) and treponemal tests should be interpreted in the same manner regardless of HIV status.

    Recommendation for the direct detection of T. pallidum by darkfield microscopy
    Darkfield microscopy should be maintained if already in use or established in sexually transmitted disease clinics where a point-of-care test for primary or secondary syphilis diagnosis would be beneficial for timely patient treatment.

    Recommendation for direct detection of T. pallidum by immunohistochemistry and silver staining
    Immunohistochemistry is preferred over silver staining for formalin-fixed, paraffin-embedded tissue sections regardless of anatomic site.

     

    References

    1. Sexually Transmitted Infections Surveillance, 2022. Centers for Disease Control and Prevention. Accessed February 8, 2024. https://www.cdc.gov/std/statistics/2022/default.htm#:~:text=In%202022%2C%20more%20than%202.5,from%20all%20STI%20prevention%20partners.
    2. Papp JR, Park IU, Fakile Y, Pereira L, Pillay A, Bolan GA. CDC Laboratory recommendations for syphilis testing, United States, 2024. MMWR Recomm Rep. 2024;73(1):1-32. doi:10.15585/mmwr.rr7301a1