Diagnosis
If the rash has appeared, identifying this disease (making a differential diagnosis) requires only a visual examination, since very few diseases produce a rash in a dermatomal pattern (see map). However, herpes simplex virus (HSV) can occasionally produce a rash in such a pattern. The Tsanck smear is helpful for diagnosing acute infection with a herpes virus, but does not distinguish between HSV and VZV.
When the rash is absent (early or late in the disease, or in the case of zoster sine herpete), herpes zoster can be difficult to diagnose. Apart from the rash, most symptoms can occur also in other conditions.
Laboratory tests are available to diagnose herpes zoster. The most popular test detects VZV-specific IgM antibody in blood; this appears only during chickenpox or herpes zoster and not while the virus is dormant. In larger laboratories, lymph collected from a blister is tested by polymerase chain reaction for VZV DNA, or examined with an electron microscope for virus particles.
In a recent study, samples of lesions on the skin, eyes, and lung from 182 patients with presumed herpes simplex or herpes zoster were tested with real-time PCR or with viral culture. In this comparison, viral culture detected VZV with only a 14.3% sensitivity, although the test was highly specific (specificity=100%). By comparison, real-time PCR resulted in 100% sensitivity and specificity. Overall testing for herpes simplex and herpes zoster using PCR showed a 60.4% improvement over viral culture.
Read more about this topic: Herpes Zoster