What are the commonly used serologic tests for syphilis? Commonly used serologic tests for syphilis include RPR, TPHA (TPPA) and FTA-ABS. When the syphilis spirochete invades the body, the infected person produces 2 types of antibodies that can be measured by serologic tests. One of these antibodies is not produced by the spirochete itself, but a non-specific reactant against lipid-like antigens is produced in the blood after infection with syphilis, and the more severe the infection, the higher the concentration of this reactant. rpr is a serologic test used to detect the non-spirochete antigens of this reactant, and is a routine test for detecting syphilis, which must be done first when syphilis is suspected. The other antibodies are mainly IgM and IgG antibodies against the syphilis spirochete, which are specific. TPHA (TPPA) and FTA-ABS are both serologic tests used to detect this syphilis spirochete antigen, which is a confirmatory test for syphilis, but even after regular treatment syphilis is cured most of them do not turn negative for life and do not require further treatment. What is the significance of a positive RPR result? ① Preliminary diagnosis of syphilis. About 50% of patients with stage I syphilis (chancre) have a positive reaction (titer ≥ 1:2), and if the chancre has lasted for several weeks, there is a high probability of a positive result. In the second stage of syphilis, almost 100% are strongly positive (titer ≥1:32). Even without treatment, it can be negative after many years. About 75% of late syphilis is positive (titer 1:4 to 1:8). The majority of latent syphilis is positive, with varying titers. Early latent syphilis has a high titer and late latent syphilis has a low titer, so early and late latent syphilis can be distinguished. ②The RPR test in cerebrospinal fluid is useful for the diagnosis of neurosyphilis. The titer of RPR generally peaks at the second stage of syphilis (above 1:32) and gradually decreases later, and if not treated, about half of the patients maintain a low titer (1:2 to 1:8) for life. If treated immediately at the early stage of syphilis, the formation of antibodies can be completely suppressed, which means that RPR can always be negative; if treated at the late stage of syphilis and at the second stage of syphilis, the already positive RPR titer can drop rapidly, and about 50% can be negative after 2-3 months, and about 85% can be negative after 5-6 months. If treatment is started only in advanced syphilis, the decrease in RPR titer is not obvious. The RPR titer decreases more slowly after treatment of latent syphilis. What does “serum fixation” mean? After regular treatment of syphilis (6 months after the end of treatment for early syphilis and 1.5 years after the end of treatment for late syphilis), although the RPR titer decreases, it does not turn negative and is often fixed at 1:1 to 1:4, which is called serum fixation. First of all, we should exclude whether the treatment is incomplete, and at the same time, we should examine in detail whether there is visceral or neurosyphilis, and we can do cerebrospinal fluid RPR examination. In addition, it should be considered whether there is a relapse or reinfection. The anti-syphilis treatment should be continued in adequate amount and duration. If the titer increases during the observation period, consider continuing anti-syphilis treatment. Is a positive syphilis seropositive enough to confirm syphilis? Not necessarily. Because a positive RPR can also occur in a small number of non-syphilis patients, this is called a “false positive reaction”. In addition to technical errors, they can be caused by other diseases or changes in physiological conditions. However, the titer is usually no more than 1:8. If TPHA (TPPA) and FTA-ABS are positive, syphilis is generally confirmed. What diseases can cause a false positive syphilis seropositivity? Infectious diseases such as rubella, chickenpox, infectious hepatitis, viral pneumonia, upper respiratory tract infection, acute bacterial endocarditis, active tuberculosis, schistosomiasis, filariasis, malaria, and some febrile diseases can cause a positive RPR, but the RPR titer rarely exceeds 1:8 and can turn negative within a few weeks after the disease subsides, rarely more than 6 months. In addition, the results of TPHA or FTA-ABS tests are often negative. Therefore, in the absence of clear clinical evidence of syphilis, it is inappropriate to rely on a positive RPR alone to confirm the diagnosis. Some connective tissue diseases and diseases with autoantibodies such as systemic lupus erythematosus and rheumatoid arthritis may also be positive for RPR and may persist for a long time, but the titer is low. A few pregnant women, elderly people and drug addicts may also show positive RPR.