A Case of False-Positive Treponema Pallidum Immunohistochemistry and Review of Syphilis Testing as It Pertains to Dermatologists.

A 76-year-old man presented with a persistent pruritic eruption initially diagnosed as Grover disease, unresponsive to topical corticosteroids, and only partially responsive to systemic steroids. During the course of several months, serial skin biopsies revealed spongiotic dermatitis with increasing eosinophilic infiltration. Despite negative serologic testing for syphilis and Lyme disease, a spirochete stain later showed slender filamentous organisms, prompting further investigation. Repeated serologies, including dilution to rule out the prozone phenomenon, remained negative. A panel of dermatopathologists re-reviewed the biopsies and raised concern for specimen contamination. The patient's final diagnosis was an idiopathic dermal hypersensitivity reaction, now improving with methotrexate. This case highlights the diagnostic challenges associated with false-positive and false-negative results in Treponema pallidum immunohistochemistry staining and syphilis serologic testing. We review the limitations and pitfalls of immunohistochemistry staining, serologic assays, and nucleic acid testing for syphilis, including cross-reactivity, contamination, and the prozone effect. Awareness of these diagnostic limitations is essential, because of misdiagnosis of syphilis can have significant clinical and public health implications.