Diagnosis & Testing
The signs and symptoms of herpes zoster are usually distinctive enough to make an accurate clinical diagnosis once the rash has appeared. However, diagnosis of herpes zoster might not be possible in the absence of a rash (i.e., before rash or in cases of zoster without rash). Herpes zoster is sometimes confused with herpes simplex, and, occasionally, with impetigo, contact dermatitis, folliculitis, scabies, insect bites, papular urticaria, candidal infection, dermatitis herpetiformitis, and drug eruptions. Herpes zoster can be more difficult to diagnose in children, younger adults, and people with compromised immune systems who are more likely to have atypical presentations.
Laboratory testing may be useful in cases with less typical clinical presentations, such as in people with suppressed immune systems who may have disseminated herpes zoster (defined as appearance of lesions outside the primary or adjacent dermatomes).
Direct fluorescent antibody staining of varcella-zoster virus (VZV)-infected cells in a scraping of cells from the base of a lesion is rapid, specific, and sensitive, but it is substantially less sensitive than polymerase chain reaction (PCR). This method can also be used on biopsy material and on eosinophilic nuclear inclusions.
Polymerase chain reaction (PCR) can be used to detect VZV DNA rapidly and sensitively in properly collected skin lesion specimens; however, PCR testing for VZV is not available in all settings. It is also possible to use PCR to distinguish between wild-type and vaccine strains of VZV.
Serologic methods may also be used for laboratory diagnosis of herpes zoster, although there are challenges to interpreting the results. Patients with herpes zoster may mount an IgM response and would be expected to mount a memory IgG response. However, a positive IgM ELISA result could be an indication of primary VZV infection, re-infection, or re-activation. It is also difficult to detect an increase in IgG for laboratory diagnosis of HZ since patients may have a high baseline antibody titer from prior varicella disease.
Tzanck smears of lesion specimens are inexpensive and can be performed at bedside, although they do not distinguish between VZV and herpes simplex virus infections.
In people with compromised immune systems, even when VZV infection is diagnosed by use of laboratory methods, it may be difficult to distinguish between variella and disseminated herpes zoster by physical examination or serological testing. In these instances, a history of VZV exposure or of a rash that began with a dermatomal pattern, along with results of VZV antibody testing at or before the time of rash onset may help guide the diagnosis.
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