Tzanck Smear

The Tzanck smear is a cytology test on cells from the base of a fresh vesicle or pustule. The material is stained, usually with Giemsa or Wright stain, and examined for multinucleated giant cells. Multinucleated giant cells suggest herpesvirus infection, including herpes simplex and varicella zoster. The test is non-specific and has low sensitivity.

It is largely replaced by PCR, which identifies the virus and its type. It remains useful only where PCR is unavailable.

Preparation and technique. The lesion is unroofed, and the base is scraped with a blade. The material is smeared onto a slide, air dried, fixed and stained, then examined under the microscope.

Interpretation.

• Positive finding: multinucleated giant cells with nuclear moulding and ground glass chromatin.
• Negative finding: does not exclude herpesvirus infection, since the sensitivity is low.

Causes of a false positive finding. Other cells with multinucleation, including some inflammatory and tumour cells, and poor staining that mimics nuclear changes.

Limitations and artefacts. The test cannot distinguish herpes simplex from varicella zoster. Sampling a crusted or healing lesion lowers the yield. Recent antiviral therapy may reduce the number of infected cells. Sampling from the wrong lesion gives a misleading result.

Follow up tests. PCR or culture identifies the virus and its type. Antiviral resistance testing is added where relevant.

Special groups. Immunosuppressed patients and neonates may have atypical presentations, and PCR is preferred in both groups.