Chickenpox laboratory findings
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Varicella (chickenpox) used to be very common in the United States before varicella vaccine became available. Health care providers could readily diagnose varicella by doing a clinical assessment. As a result, testing specimens and getting laboratory confirmation of varicella-zoster virus (VZV) were not usually needed. However, clinical diagnosis is becoming more challenging because fewer people get varicella. Also, varicella in vaccinated people is often mild and atypical in presentation. Therefore, laboratory confirmation of varicella is becoming increasingly important in routine clinical practice.
Laboratory testing is also recommended to:
- Confirm varicella as the cause of outbreaks
- Establish varicella as a cause of death
- Determine susceptibility to varicella
- Finally, specialized laboratory testing can be used to determine if suspected vaccine-related adverse events were caused by vaccine-strain VZV.
Determining Chickenpox Susceptibility
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 is done on blood samples. It can readily detect seroconversion to a natural infection with VZV. However, it is not sensitive enough to detect all seroconversions after vaccination. The more sensitive purified glycoprotein ELISA (gpELISA) or fluorescent antibody to membrane antigen (FAMA) tests have been used in research settings to detect seroconversion after vaccination. However, these are not available commercially.
IgG avidity has been used in research settings to determine if a person who is IgG positive for VZV was infected with the virus in the past or more recently. Avidity testing can be used to determine if the most recent VZV rash was due to primary infection (varicella) or reactivation (herpes zoster). People infected in the past tend to have antibodies with high affinity for binding to the antigen compared with people with a more recent infection who have a low affinity. As a result, low avidity is an indicator for VZV primary infection. This test is not available commercially.
Laboratory Confirmation of Vaccine-Adverse Event
PCR genotyping of skin lesions can be used to distinguish rashes due to wild-type VZV from rashes caused by vaccine type (Oka)-strain VZV. Samples of blood, cerebrospinal fluid, or biopsy specimens may also be tested for vaccine-strain VZV to confirm a vaccine-adverse event. VZV-strain-specific testing is not available commercially; it is available at some laboratories, including the CDC National VZV Laboratory. Examples of possible varicella vaccine-adverse events for which PCR genotyping for VZV can be useful include varicella in a vaccinee 7–42 days after vaccination, herpes zoster in a vaccinee >42 days post-vaccination, and suspected secondary vaccine-strain VZV transmission.
Interpretation of Laboratory Findings
Confirmation of Suspected Varicella
The most sensitive method for confirming a diagnosis of varicella is the use of PCR to detect VZV in skin lesions (vesicles, scabs, maculopapular lesions). Vesicular lesions, if present, are the best for sampling. Adequate collection of specimens from maculopapular lesions in vaccinated persons can be challenging. However a recent study comparing a variety of specimens from the same patients suggests that maculopapular lesions collected with proper technique can be highly reliable specimen types for detecting VZV.
IgM testing is considerably less sensitive than PCR testing of skin lesions. Commercial IgM assay may not be reliable and false negative IgM results are not uncommon. A positive IgM ELISA result, although suggestive of a primary infection, does not exclude re-infection or reactivation of latent VZV. Careful clinical evaluation may be needed to determine if a rash is varicella or herpes zoster. Nevertheless, IgM testing is readily available and a positive result from a person with a rash is usually interpreted as laboratory confirmation of varicella. A single positive IgG ELISA result cannot be used to confirm a varicella case.
Paired acute and convalescent sera showing a four-fold rise in IgG antibodies have excellent specificity for varicella, but are not as sensitive as PCR of skin lesions for diagnosing varicella. Persons with prior history of vaccination or disease history 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.
Establishing Laboratory Evidence of Immunity to Varicella
A positive IgG ELISA result indicates that a person has antibodies to VZV either from past varicella disease history or vaccination. This test cannot distinguish whether the antibodies were from a past episode of varicella or vaccination. It is a fairly sensitive and specific assay. However, none of the currently available commercial assays are sufficiently sensitive to detect immunity in all vaccinated persons.
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