Development of a real-time PCR assay to detect Treponema pallidum in clinical specimens and assessment of the assay's performance by comparison with serological testing

The incidence of infectious syphilis in men who have sex with men and human immunodeficiency virus-infected patients has increased steadily in Victoria, Australia, since 2002. A TaqMan real-time PCR assay targeting the polA gene of Treponema pallidum (TpPCR) was developed. The analytical sensitivity of the assay was estimated to be 1.75 target copies per reaction. Initially, the assay was used to test a variety of specimens (excluding blood) from 598 patients. Of the 660 tests performed, positive PCR results were obtained for 55 patients. TpPCR results were compared with serology results for 301 patients being investigated for early syphilis. Of these patients, 41 were positive by both TpPCR and serology, 246 were negative by both TpPCR and serology, 4 were TpPCR positive but negative by serology, and 10 were TpPCR negative but showed evidence of recent or active infection by serology. Directly compared with serology, TpPCR showed 95% agreement, with a sensitivity of 80.39% and a specificity of 98.40%. Potential factors leading to the discrepant results are discussed. Concurrent serology on 21 patients with TpPCR-positive primary syphilitic lesions demonstrated that a panel of current syphilis serological tests has high sensitivity for the detection of early syphilis. We found that TpPCR is a useful addition to serology for the diagnosis of infectious syphilis. Direct comparison with other T. pallidum PCR assays will be required to fully assess the limitations of the assay.