Varicella Diagnostics

Laboratory Tests and Ancillaries

Varicella  

Laboratory tests are not required for most cases of varicella but may be necessary in subsequent episodes, atypical clinical presentation or cases of drug resistance.  A skin specimen obtained by unroofing a fresh, fluid-filled vesicle and rubbing the base of the lesion with a polyester swab is used for confirmation of varicella while a blood specimen is used to test for immunity. Confirmatory tests either demonstrate viral DNA or protein through polymerase chain reaction (PCR) and direct fluorescent antibody (DFA) or isolate the virus through viral culture.

Viral Culture  

Newer techniques for viral culture provide results within 2-3 days. These are not highly sensitive due to the high lability of VZV and are mostly done for scientific/clinical purposes. 

Tzanck Smear  

This is convenient to perform but will not differentiate VZV from herpes simplex virus (HSV).  

Polymerase Chain Reaction (PCR)  

Vesicular swabs or scrapings and scabs from crusted lesions can be used as specimens to identify VZV DNA. It is considered the gold standard for diagnosing varicella due to high specificity and sensitivity. PCR enables rapid clinical diagnosis by providing results within hours. For patients with suspected central nervous system (CNS) involvement, VZV PCR may be performed using cerebrospinal fluid (CSF).  

Serology  

Serological tests may be used as confirmatory tests based on the presence of IgM or a fourfold rise in acute- and convalescent-phase IgG antibodies to VZV but are less reliable than PCR and immunofluorescence due to lack of sensitivity and specificity. The complement fixation test requires paired sera and cross-reacts with HSV.

Direct and Indirect Immunofluorescence

Fluorescence microscopy rapidly identifies the presence of VZV proteins in cells scraped from lesions. This may be an alternative to PCR but is less sensitive and requires skill in obtaining and handling specimens.  

Herpes Zoster  

Clinical diagnosis is sufficient in the case of typical herpes zoster, but diagnostic testing is useful for differentiating herpes zoster from herpes simplex if organ involvement is suspected and for atypical presentation. Swabs from fresh lesions or tissue biopsies can be submitted for viral culture. Direct fluorescent antigen testing or PCR should be done for uncertain diagnoses and atypical lesions. Scabs are very good specimens for PCR testing. PCR is the most sensitive and specific method for the diagnosis of VZV infection. Culture, serology, and direct and indirect immunofluorescence studies are the same with varicella. Serologic test for VZV-specific IgM, IgG and IgA in patients with herpes zoster-like pain and zoster-associated (facial nerve) palsy may be considered.