Contact dermatitis, herpes simplex comprise the differential
The differential diagnosis includes contact dermatitis, which is often associated with exposure to poison ivy. The vesicles of contact dermatitis follow the pattern of exposure, often forming a linear pattern, and are not distributed along a dermatome.4
Herpes simplex, caused by HSV-1 or HSV-2, may present with a vesicular rash similar to HZ. The rash does not follow the dermatomal distribution and the vesicles are more uniform than is seen in HZ. The rash may periodically recur, which confirms the diagnosis.1
When lab testing is helpful
Generally speaking, diagnosis of HZ does not require lab work, but tests may be helpful with atypical or complicated presentations. Direct fluorescent antibody testing of scrapings or viral specimens can demonstrate specific antigens for a relatively quick and inexpensive confirmation of VZV.
Polymerase chain reaction (PCR) testing is superior to culture for definitive identification and can distinguish between wild-type and the Oka (vaccine)-type varicella.5 Serology is available, but is used more to identify those at risk for infection than to assist with diagnosis.
With our patient, the diagnosis of HZ was evident by the history and physical exam. Testing was performed with the assistance of our microbiology lab to aid in postmarketing surveillance of the varicella vaccine. Samples, obtained from aspiration of vesicular fluid and swabs of unroofed vesicles, were placed in viral culture media and sent to the Centers for Disease Control and Prevention for PCR. The results were genotype specific for vaccine type VZV.