A Tzanck preparation is a long-used bedside procedure that takes only a few minutes to perform and is positive in cases of HSV1, HSV2, or varicella-zoster virus (VZV) infection. The procedure does not differentiate among the three viruses. However, HSV infection can be distinguished from varicella clinically. The procedure is done by unroofing a vesicle and scraping its base with a no. 15 blade scalpel. The scrapings are placed on a glass slide and allowed to air dry for 1 to 2 minutes. A blue stain such as Giemsa or toluidine blue is applied for 60 seconds and then gently rinsed off. The slide is dried, mineral oil is applied, and the preparation is covered with a microscope cover slip. It is then ready to be viewed. Multinucleated giant cells are readily seen throughout the sample, confirming the viral etiology of the blister.
Rapid immunostaining is available and can be used with high sensitivity and specificity to diagnose and differentiate the various herpesvirus types. This form of direct fluorescent antibody (DFA) testing is similar to the Tzanck preparation. As in the Tzanck preparation, scrapings of the blister base are placed on a glass microscope slide. The slide is stained with antibodies corresponding to the various herpesviruses. The sample is viewed under fluorescent microscopy, and a positive sample fluoresces with one of the specific viral stains. This test takes 1 to 2 hours to perform.
Viral tissue cultures can also be performed to differentiate the HSV types, but the results can take days to 1 week to obtain. This is the most sensitive and specific test for the infection.
Histology: Examination of a biopsy specimen of a blister shows ballooning degeneration of the epidermal keratinocytes. This degeneration forms the blister cavity. There is a mixed inflammatory infiltrate around the superficial and deep dermal vascular plexus. Multinucleated giant cells are found at the base of the blister pocket. The skin biopsy findings are unable to differentiate HSV1 from HSV2 or from VZV infection.
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