MMWR
CDC releases lab guidance for syphilis testing
CDC’s new recommendations are intended for use by clinical lab directors, lab 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 lab reporting, and counsel and treat patients.
Key recommendations
- 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., RPR and VDRL). Reports should not contain mathematical symbols such as > or < signs.
- 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 lab resources, including staff, space and costs, test volume, and patient populations served.
- 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 Treponema pallidum particle agglutination test is the preferred manual treponemal test.
- Syphilis serologic testing in pregnant persons. Nontreponemal (lipoidal antigen) and treponemal tests should be interpreted in the same manner regardless of pregnancy status.
- 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.
- Direct detection of Treponema pallidum by darkfield microscopy. Darkfield microscopy should be maintained if already in use or established in STI clinics where a point-of-care test for primary or secondary syphilis diagnosis would be beneficial for timely patient treatment.
- Direct detection of Treponema pallidum by immunohistochemistry and silver staining. Immunohistochemistry is preferred over silver staining for formalin-fixed, paraffin-embedded tissue sections regardless of anatomic site.
Source:
Papp JR, et al. (2024, February 8). MMWR. CDC Laboratory Recommendations for Syphilis Testing, United States, 2024. https://www.cdc.gov/mmwr/volumes/73/rr/rr7301a1.htm