Kayla Scully¹, Jessica Gartside², Kyung Park¹

¹Pathology Queensland, Gold Coast University Hospital, Queensland, Australia; ²Queensland Health, Gold Coast University Hospital, Queensland, Australia 

Histological diagnosis of syphilis has improved dramatically in recent times since the discovery and widespread implementation of immunohistochemical stains specific for the causative organism Treponema pallidum. Immunohistochemistry (IHC) has been repeatedly shown to be more sensitive and specific than previous silver-based staining methods for identifying T. pallidum on light microscopy. Despite the high specificity of IHC, false positives due to cross-reactivity with other organisms are known to occur. This case report discusses a patient with a non-healing ulcer who attracted a working diagnosis of syphilis after a biopsy showed positive T. pallidum IHC staining due to cross-reactivity with Corynebacterium macclintockiae. While IHC remains a reliable tool in the diagnosis of syphilitic lesions, this case demonstrates the potential pitfalls and highlights the need for secondary testing utilising serological and/or molecular methods to minimise the risk of false positives.  

The above work was an original idea of the presenting author with contributions from the additional authors. No generative AI was used in this case.