There are two different types of serologic assays for syphilis; nontreponemal and treponemal. A nontreponemal assay (traditional algorithm), such as the rapid plasma reagin (RPR) test, is an indicator of an active or recent syphilis infection and can be used to monitor response to therapy over time. The nontreponemal testing algorithm is recommended by the CDC, preferred by doctors and used in a majority of labs for syphilis screening.
A treponemal test (reverse algorithm) cannot differentiate between active or previously treated infection, which can create a false-positive result. The immune response to syphilis creates antibodies to fight the infection; those antibodies remain in the blood after successful treatment, because treponemal tests cannot monitor the response to therapy.
“The CDC continues to recommend the traditional screening algorithm using a nontreponemal test (e.g., RPR or VDRL), with reactive nontreponemal tests confirmed by treponemal testing.”
Gail Bolan, MD; Director, Division of STD Prevention; National Center For HIV/AIDs, Viral Hepatitis, STD, and TB Prevention