Lab Testing for Varicella-Zoster Virus: PCR, Serology, and Vaccine Versus Wild Strains

The main points

• This guidance covers testing for primary infection with varicella-zoster virus (VZV) — varicella, or chickenpox — and its reactivation, herpes zoster (shingles).
• PCR (polymerase chain reaction) is the most helpful test for confirming both.
• The CDC Herpesvirus Laboratory offers several VZV-specific tests.

Varicella-zoster virus in tissue culture, under the microscope. Image from CDC's page.

When to test

Testing helps most when a case doesn't look typical. It is recommended to:

• confirm suspected varicella, and varicella as the cause of an outbreak;
• confirm varicella in severe (hospitalization or death) or unusual cases;
• find out whether someone is susceptible;
• decide whether a suspected vaccine adverse event was caused by the vaccine strain.

The CDC Herpesvirus Laboratory

It tests free of charge for state and local health departments that need confirmation of VZV infection or of atypical zoster, and for physicians and scientists in certain epidemiologic and laboratory studies. It also takes specimens for suspected vaccine adverse events:

• a rash 7 to 42 days after vaccination;
• suspected spread of the vaccine virus to someone else;
• zoster in a vaccinated person;
• any serious adverse event.

For specimen collection, storage and handling, contact the laboratory at CDC, 1600 Clifton Road NE, Atlanta, GA 30333 (404-639-5455 or 404-639-6403), and see CDC's guide to collecting and shipping VZV specimens.

PCR: the test of choice

PCR detects VZV DNA quickly and sensitively in skin lesions — blisters, scabs, and flat or raised spots — and real-time PCR can be done within a day. For unvaccinated and vaccinated people alike, it is the most reliable confirmation.

• Best specimens: blisters or scabs, when present.
• Flat or raised spots (maculopapular lesions) in vaccinated people are harder to sample well. But one study comparing specimens from the same patients, vaccinated with one dose, found that such lesions collected properly are highly reliable.
• Poor specimens: nasopharyngeal secretions, saliva, urine, bronchial washings and cerebrospinal fluid rarely give an adequate sample and often produce false negatives.

Tests that fall short

• Direct fluorescent antibody (DFA) and viral culture are less sensitive than PCR and generally not recommended; culture is also slower.
• A Tzanck smear is quick but insensitive and not specific to VZV.
• Serology has little use in confirming zoster and should be used only when no specimen suitable for PCR exists.
• • IgM tests are much less sensitive than PCR of lesions and prone to poor specificity. A positive IgM points to recent active infection but cannot separate primary infection from reinfection or reactivation, because IgM is made with every exposure.
  • Paired acute and convalescent sera showing a fourfold rise in IgG are highly specific for varicella, but less sensitive than PCR; people with past vaccination or disease may start so high that they never show a fourfold rise, and the method takes two visits. A rise is also hard to detect in zoster.
  • A single positive IgG ELISA cannot confirm a case.

Laboratory criteria for diagnosis

• Preferred: VZV DNA by PCR from a clinical specimen — ideally scabs, blister fluid, or cells from the base of a lesion. PCR also confirms breakthrough varicella.
• A positive IgM test when a varicella-like rash is present.
• A fourfold or greater rise in IgG between acute and convalescent sera, by any standard assay.

More: the lab-testing section of CDC's varicella surveillance manual.

Vaccine strain or wild virus?

PCR and genotyping can tell wild-type VZV from the vaccine's Oka/Merck strain. That matters for suspected vaccine adverse events such as:

• varicella, or a complication of it, 7 to 42 days after vaccination;
• zoster in a vaccinated person;
• suspected spread of the vaccine strain.

Blood, cerebrospinal fluid, biopsy or autopsy specimens can also be tested to settle the cause, though they can give false negatives. The CDC laboratory and the APHL Vaccine Preventable Diseases Reference Centers — in Wisconsin, California, New York and Minnesota, each serving designated states — distinguish the strains by strain-differential real-time PCR and by PCR with restriction fragment length polymorphism (quick reference guide).

Testing for immunity

• IgG ELISA: a single test shows whether someone has antibodies — from past disease or vaccination, without telling which — or might need varicella-zoster immune globulin (in the U.S., VariZIG). Commercial ELISAs are recommended for screening, most often the whole infected cell (wc) ELISA, which readily detects antibodies after natural infection. Their sensitivity and specificity vary widely, and some are unreliable even for past disease.
• After vaccination, routine immunity testing is not needed: commercial IgG assays are not sensitive enough to catch every response to the vaccine. The more sensitive glycoprotein ELISA (gpELISA) is used only in research.
• IgG avidity, a CDC-developed research test, shows whether the latest VZV rash was primary infection (low avidity) or reactivation (high avidity, a sign of infection long ago). Vaccinated people develop moderate to high avidity, so the test is unlikely to separate old infection or vaccination from a recent breakthrough case. It is not sold commercially.

More from CDC: MMRV testing recommendations for clinicians and Varicella and Breakthrough Varicella: To Test or Not to Test.

Sources

Based on "Laboratory Testing for Varicella-Zoster Virus (VZV)," Chickenpox (Varicella), Centers for Disease Control and Prevention; a work of the United States government in the public domain.