Key points
- This varicella-zoster virus (VZV) laboratory testing information applies to testing and diagnosis of primary VZV infection (varicella) and reactivation (herpes zoster).
- Polymerase chain reaction (PCR) is the most helpful laboratory test for confirming cases of varicella and herpes zoster.
- The CDC Herpesvirus Laboratory can provide several types of VZV-specific testing.
Overview
Laboratory testing can be useful, particularly in cases with less typical clinical presentations.
Testing is recommended to:
- Confirm suspected cases of varicella.
- Confirm varicella as the cause of outbreaks.
- Confirm varicella in severe cases (hospitalizations or deaths) or unusual cases.
- Determine susceptibility to varicella.
- Determine if suspected vaccine-related adverse events were caused by vaccine-strain VZV.
Resource lab
CDC Herpesvirus Laboratory:
The CDC Herpesvirus Laboratory can provide several types of VZV-specific testing free-of-charge to state and local public health departments who require confirmatory evidence for VZV infection or confirmation of atypical herpes zoster cases. The laboratory also conducts free testing for physicians and scientists participating in various epidemiologic and laboratory-based studies.
Specimens can also be submitted for suspected vaccine-related adverse events, including:
- Rash occurring 7 to 42 days after vaccination
- Suspected secondary transmission of the vaccine virus
- Herpes zoster in a vaccinated person
- Any serious adverse event
For more information on VZV specimen collection, storage, and handling, please contact the CDC Herpesvirus Laboratory.
Contact information
Centers for Disease Control and Prevention
1600 Clifton Road, NE
Atlanta, GA 30333
Tel: 404-639-5455 or 404-639-6403
Types of tests
Testing recommendations for varicella
Polymerase chain reaction (PCR)
PCR is the most useful laboratory test for confirming suspected varicella and herpes zoster. PCR can detect VZV DNA rapidly and sensitively in skin lesions (vesicles, scabs, maculopapular lesions).
Specimens for PCR testing
- Vesicular lesions or scabs, if present, are the best for sampling.
- Adequate collection of specimens from maculopapular lesions in vaccinated people can be a challenge.
- However, one study1 that compares a variety of specimens from the same patients vaccinated with one dose suggests that maculopapular lesions collected with proper technique are a highly reliable specimen types for the detection of VZV.
- However, one study1 that compares a variety of specimens from the same patients vaccinated with one dose suggests that maculopapular lesions collected with proper technique are a highly reliable specimen types for the detection of VZV.
- Other sources, such as nasopharyngeal secretions, saliva, urine, bronchial washings, and cerebrospinal fluid are less likely to provide an adequate sample; they often lead to false negative results.
DFA tests, viral culture, and Tzanck smears
Other viral isolation techniques for the confirmation of varicella are direct fluorescent antibody assay (DFA) and viral culture. However, these techniques are generally not recommended because they are less sensitive than PCR. In the case of viral culture, these techniques take longer to generate results.
Like DFA, a Tzanck smear has a rapid turnaround time but is not recommended because of its limited sensitivity and is not specific for VZV. Moreover, real-time PCR protocols can be completed within one day.
Serologic tests
Serologic methods have limited use for laboratory confirmation of herpes zoster and should only be used when suitable specimens for PCR testing are not available.
IgM serologic testing is considerably less sensitive than PCR testing of skin lesions. IgM serology provides evidence for a recent active VZV infection. However, it cannot distinguish between primary infection and reinfection or reactivation from latency. This is because specific IgM antibodies are produced with each exposure to VZV. IgM tests are also inherently prone to poor specificity.
Specimens for serologic testing
Measuring acute and convalescent sera also has limited value, since it is difficult to detect an increase in IgG for laboratory diagnosis of herpes zoster.
Paired acute and convalescent sera showing a four-fold rise in IgG antibodies have excellent specificity for varicella. However, they are not as sensitive as PCR of skin lesions for the diagnosis of varicella. People with a prior history of vaccination or disease may have very high baseline titers and may not achieve a four-fold increase in the convalescent sera. The usefulness of this method for diagnosing varicella is further limited as it requires two office visits. A single positive IgG ELISA result cannot confirm a varicella case.
Laboratory guidelines
- The preferred diagnosis method is a demonstration of VZV DNA by PCR tests from a clinical specimen, ideally scabs, vesicular fluid, or cells from the base of a lesion.
- PCR is also useful for confirming breakthrough varicella. Other methods, such as DFA and culture, are available for diagnosis but are less sensitive and specific than PCR.
- Positive serologic test for varicella-zoster immunoglobulin M (IgM) antibody when a varicella-like rash is present.
- Four-fold or greater rise in serum varicella immunoglobulin G (IgG) antibody titer by any standard serologic assay between acute and convalescent sera.
For both unvaccinated and vaccinated people, PCR is the most reliable method for confirming a VZV infection.
Specimen collection
Learn about guidelines for collecting and shipping specimens for VZV testing (varicella and herpes zoster). These include methods for VZV serologic assays and VZV PCR/genotyping, sources for suitable supplies, and how to submit specimen to CDC.
Virus strain identification
PCR testing and genotyping can distinguish between wild-type VZV and vaccine-type (Oka/Merck) strains of VZV.
Such testing is used in situations when it is important to recognize the two VZV strains, like in suspected vaccine adverse events. Examples of possible varicella vaccine-adverse events include:
- Varicella or a varicella-related complication in a vaccinated person 7 to 42 days after vaccination.
- Herpes zoster in a vaccinated person.
- Suspected secondary vaccine-strain VZV transmission.
Samples of blood, cerebrospinal fluid, biopsy, or autopsy specimens may also be tested for vaccine-strain/wild-type VZV discrimination to confirm etiology and to identify a vaccine-adverse event. However, these specimens and can lead to false negative results.
Specialized labs
Evaluating VZV susceptibility
IgG ELISA
A single serologic IgG test can determine if a person has antibodies to VZV from past varicella disease or who may be candidates for varicella-zoster immune globulin (VZIG). The product available in the United States is VariZIG.
Commercial enzyme-linked immunosorbent assays (ELISAs) are the recommendation for screening. Whole infected cell (wc) ELISA is the most commonly used test to determine if a person has antibodies to VZV from past varicella disease. Wc ELISA taken from blood samples can readily detect seroconversion to natural infection with VZV.
Routine testing for varicella immunity following vaccination is unnecessary because commercially available VZV IgG assays are not sensitive enough to detect all seroconversions after a vaccination.
The more sensitive purified glycoprotein ELISA (gpELISA) has been used in research settings to detect seroconversion after vaccination. However, testing with gpELISA is not available commercially.
IgG avidity
IgG avidity in research settings determines if a person who is IgG positive for VZV was infected with the virus in the past or more recently.
The laboratory at CDC developed an IgG avidity assay, which determines if the most recent VZV rash was due to primary infection (varicella) or reactivation (herpes zoster).
- High avidity is an indicator of a remote infection.
- Low avidity is an indicator of VZV primary infection.
People infected in the past tend to have antibodies with a high affinity for binding to the antigen. People with a more recent infection tend to have a low affinity.
Vaccinated people undergo antibody affinity maturation following vaccination, which leads to moderate to high IgG avidity VZV antibody. Measurements of VZV avidity in vaccinated people are unlikely to distinguish remote infection (or vaccination) from recent (breakthrough) infection with VZV. This test is not available commercially.
Establishing lab evidence of immunity to varicella
IgG ELISA
A positive IgG ELISA result indicates that a person has antibodies to VZV from past varicella disease or vaccination. This test cannot distinguish whether the antibodies were from a past episode of varicella or vaccination.
While many commercial VZV IgG ELISAs perform well enough to detect seroconversion for infection by wild-type viruses, the performance specifications (specificity and sensitivity) of these methods vary widely.
IgG assays
Some commercially available VZV IgG assays are unreliable, even for detecting a natural disease history in people. Currently, commercial VZV IgG methods sensitive and specific enough to reliably detect seroconversion to the vaccine are unavailable.