Serology of Treponema pallidum infection: methodological considerations and seroprevalence in selected Italian populations

An increase of syphilis incidence in recent years has been reported throughout Europe and also in Italy. The advent of automated serological assays for the detection of anti-Treopnema pallidum antibodies increases the sensitivity and the reproducibility of antibody detection: one of such methods (the chemiluminescent Architect Syphilis TP assay) has been evaluated in a multisite study, that involved 22 Italian laboratories, on 62,036 serum specimens obtained from blood donors (n= 44,884), from hospital in- or outpatients at low (n= 16,482) or high (n= 284) risk for treponemal infection and from 386 patients already diagnosed with syphilis at different stages. The Architect Syphilis assay showed a good reproducibility both on controls (718 replicates, CV of 14.4% on negative and of 12.7% on positive control) and on clinical specimens (93 samples, CV of 3.61%). The sensitivity on syphilis cases was 99.73% compared to 99.43% by other EIAs, and the specificity on 44,898 blood donors was 99.67%.The positive predictive value was related to the signal of the Architect assay (4.3% for sample to cutoff (S/CO) values 3). The positivity rates for anti-Treponema pallidum antibodies were 0.15% in blood donors, 3.46% among low risk subjects and 16.90% among patients at high risk for syphilis infection. The high prevalence in low risk groups enhances the need for a surveillance of syphilis, that appears to increase in the general population, with sensitive first-level assays. An appropriate algorithm for confirmation of low positive results by Western blot testing should be envisioned.