Pros and Cons of serological testing in syphilis diagnosis and follow up

Since a proper diagnosis of syphilis is often difficult due to the wide variability of both clinical picture and laboratory test results, early recognition of infection caused by Treponema pallidum is crucial for a timely and effective treatment. In most cases, definitive diagnosis relies upon serological testing. A screening ELISA test, coupled with a quantitative RPR test and specific IgM antibodies detection, is currently regarded as the basic diagnostic procedure. In addition, a quantitative particle agglutination TP-PA test, FTA-abs IgG test and, eventually, a western-blot IgG and IgM test, allow to achieve a whole serological pattern for each patient at the time of first diagnosis. In this study, a group of serum samples (n=107) and cerebro-spinal fluid (n=3) were retrospectively analyzed using the above mentioned tests. A population of 19 patients whose clinical picture was unremarkable for syphilis, showed border-line values at screening and negative results on confirmation tests. Thirty-three out of 91 luetic patients were diagnosed as primary or early secondary syphilis, 36 as latent syphilis, 3 as neurosyphilis, and 3 were neonates with passive specific immunization. Quantitative RPR test and detection of specific IgM antibodies exhibited extremely high values in all 33 primary syphilis patients; a whole positive luetic pattern was also obtained by confirmation tests. Searching for IgM antibodies, a capture elisa test compared with a single device rapid elisa test showed an overall concordance of 98.1%. In luetic patients other than primary syphilis, quantitative RPR test and detection of specific IgM antibodies provided less relevant values and a low prevalence pattern, whereas TP-PA and FTA-abs tests showed persistent positives results. In the follow up of 19 initially treated patients, quantitative RPR values and specific IgM antibodies index showed a slow, progressive decrease until negative. Conclusion: a comprehensive initial sieroluetic framework is a relevant diagnostic tool for primary diagnosis; along the follow up, elisa screening test,TP-PA and FTA-abs IgG have a poor clinical significance; instead, quantitative RPR titers and specific IgM antibody index change over time, and as such, they are reliable tools to evaluate the therapeutic efficacy of treatment and a rational management of luetic patients.