Tzanck smear

The Tzanck smear is performed by unroofing a fresh (not crusted) vesicle, scraping its base with a scalpel blade, smearing the material onto a glass slide, and staining with Giemsa, Wright, or Papanicolaou stain. The test is positive when multinucleated giant keratinocytes (Tzanck cells) with ground-glass nuclei are seen, indicating cytopathic viral effect from herpesviruses (HSV-1, HSV-2, or VZV). Acantholytic cells may also be seen in pemphigus vulgaris, as the test can detect acantholysis in addition to viral cytopathic changes. Clinically, the Tzanck smear is a rapid, inexpensive, point-of-care test useful in diagnosing herpetic infections (e.g., herpes labialis, genital herpes, eczema herpeticum) and varicella/zoster, but it has largely been supplanted by more specific tests such as PCR, direct fluorescent antibody (DFA) testing, or viral culture, especially since the Tzanck smear cannot differentiate between HSV and VZV, nor determine HSV subtype. In pemphigus vulgaris, the Tzanck smear shows acantholytic cells (rounded, detached keratinocytes) reflecting loss of desmosomal adhesion, which can support a clinical diagnosis pending biopsy and direct immunofluorescence. Board relevance: know that a positive Tzanck smear showing multinucleated giant cells is classic for herpesvirus infections, while acantholytic cells are seen in pemphigus vulgaris; recognize its limitations compared to PCR/DFA.