By Ethan Covey
In response to increasing U.S. syphilis cases, the CDC has released its first laboratory testing recommendations for the disease.
The recommendations are “intended for use by clinical laboratory directors, laboratory 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” (MMWR Recomm Rep 2024;73[1]:1-32).
“The U.S. is experiencing a syphilis epidemic with sustained increases in the number of cases reported to the CDC yearly,” said John R. Papp, PhD, a health scientist in the CDC’s Division of STD Prevention, National Center for HIV, Viral Hepatitis, STD, and TB Prevention. “Laboratories have a critical role in the public health response to the syphilis epidemic.
“The responsibility of the laboratory is to test specimens and report results in a timely manner, allowing clinicians to efficiently make clinical diagnoses for patient management,” Dr. Papp added. “These lab recommendations for syphilis testing are the first published by the CDC, and they set a baseline for syphilis testing in the U.S.”
Cases of primary and secondary syphilis have risen from 5,979 cases in 2000 to 133,945 reported in 2020, a 2,140% increase.
The recommendations include information about laboratory-based tests, point-of-care (POC) tests, processing of samples and reporting test results to aid laboratorians and clinicians in the diagnosis of syphilis and to provide a necessary update.
“Most serologic-based testing for syphilis, which are tests that detect antibodies to the causative bacterium for syphilis, Treponema pallidum, are based on principles learned over 50 years ago,” Dr. Papp said. “The terminology was based on the understanding of the contemporaneous scientific findings and became the basis for which to describe the serologic testing concepts for syphilis that are still used today. These recommendations revisited the terminology to be consistent with the scientific evidence related to the immunobiology of T. pallidum.”
Along with highlighting the need for direct detection and POC tests, the recommendations provide details regarding serologic testing and how to navigate its usage to effectively diagnose syphilis and initiate the correct therapy.
“Clinical features in adults progress through different stages that require different treatment regimens,” Dr. Papp said. “Healthcare providers use laboratory results from serologic tests to appropriately stage syphilis cases to ensure appropriate use of antimicrobials. However, serologic testing is complicated, and laboratories must use serologic tests that detect different antibodies to T. pallidum so that results can be better used by a healthcare provider when evaluating a patient for syphilis.
“Specimens should be diluted in some serologic test types to reach an end point, that is, when no further antibodies are detected in the sample, so that the result represents the level of antibodies the patient has to syphilis,” he added. “This is important because these antibody levels will drop if the patient is effectively treated, and healthcare providers use these results to monitor if the patient is cured from syphilis. There are different approaches as to the sequence of serologic tests, and the recommendations provide information for laboratories to consider when deciding on the most suitable approach.”
The CDC aims to revise the recommendations as needed based on new research or technological advancements for syphilis clinical laboratory science.
“Additional innovation, research and investment in diagnostics are critical to prevent and control STIs [sexually transmitted infections],” Dr. Papp said. “Equitably stopping the syphilis epidemic and addressing the nation’s broader STI challenges requires a coordinated and sustained effort shared by federal leadership, state and local health departments, healthcare systems, providers, and private industry.”
