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

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Claudio Galli *
Pierpaolo Petasecca Donati
Gruppo Italiano Studio Sierologia Infezione Luetica
(*) Corresponding Author:
Claudio Galli |


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.

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