PCR Testing for Treponema Pallidum in Paraffin-embedded Skin Biopsy Specimens: Test Design and Impact on the Diagnosis of Syphilis

Background: Syphilis, a chronic infection caused by Treponema pallidum, is a disease which is increasing in incidence, and thus more and more becoming a differential diagnosis in routine pathology.

Aims: To develop a PCR-based molecular assay for the detection of T pallidum in formalin-fixed, paraffin-embedded tissues, and evaluate its diagnostic power, especially in comparison with other ancillary methods (immunohistochemistry and Dieterle staining).

Methods: 36 skin biopsy specimens with the clinical and/or serological diagnosis of syphilis were evaluated by morphology, immunohistochemistry and silver staining. A semi-nested PCR assay targeting the T pallidum DNA polymerase A gene was designed and applied. Specificity of amplification was confirmed by direct sequencing of PCR products.

Results: Overall, the presence of T pallidum was detected in skin biopsy specimens in 20 cases, by immunohistochemistry, Dieterle staining, or PCR. Immunohistochemistry testing was positive in 17/35 cases tested, and Dieterle staining in 9/35 cases tested. In comparison, PCR testing was positive in 14/36 cases, and highly dependent on the tissue quality. When excluding cases with compromised DNA quality, PCR testing was positive in all cases except one, including three cases negative by immunohistochemistry and Dieterle staining.

Conclusions: PCR testing is significantly more sensitive than silver staining, and provided that DNA quality is sufficient, at least as sensitive as immunohistochemistry for the detection of T pallidum in formalin-fixed, paraffin-embedded skin biopsy specimens. Therefore, it is a useful ancillary tool in the histological diagnosis of syphilis.