How is it used?
Active cases of chickenpox and shingles, which are caused by the varicella zoster virus (VZV), are usually diagnosed based upon the person’s symptoms and clinical presentation. Most adults have been infected with VZV, and children are now
vaccinated; therefore, general population screening is not done. However, testing for VZV or for the
antibodies produced in response to VZV infection may be performed in certain cases. For example, it may sometimes be performed in pregnant women, in newborns, in patients prior to organ transplantation, and in those with
HIV/AIDS. The goals for testing may include:
- to determine if someone has been previously exposed to VZV either through past infection or vaccination and has developed immunity to the disease
- to distinguish between an active or prior infection
- to determine whether someone with severe or atypical symptoms has an active VZV infection or has another condition with similar symptoms
There are several methods of testing for VZV:
Antibody testing
When you are exposed to VZV, your immune system responds by producing antibodies to the virus. Two types of VZV antibodies may be found in the blood: IgM and IgG. IgM antibodies are the first to be produced by the body in response to a VZV infection. They are present in most individuals within a week or two after the initial exposure. IgM antibody production rises for a short time period and declines. Eventually, the level (titer) of VZV IgM antibody usually falls below detectable levels. Additional IgM may be produced when latent VZV is reactivated. IgG antibodies are produced by the body several weeks after the initial VZV infection to provide long-term protection. Levels of IgG rise during the active infection, then stabilize as the VZV infection resolves and the virus becomes inactive. Once a person has been exposed to VZV, they will have some measurable amount of VZV IgG antibody in their blood for the rest of their life. VZV IgG antibody testing can be used, along with IgM testing, to help confirm the presence of a recent or previous VZV infection.
Viral detection
Viral detection involves finding VZV in a blood, fluid, or tissue sample. This can be done either by culturing the virus or by detecting the virus’s genetic material (VZV DNA).
- VZV culture—a sample of fluid is collected from a vesicle (the most common sample). It is incubated in a culture of live cells and nutrients to grow and isolate the virus. This test is sensitive and specific, but it takes 2 or more days to complete. Fresh lesions are the best for this test. Viral shedding decreases over time and can lead to a false negative result.
- VZV DNA testing–performed to detect VZV genetic material in a patient sample. This method is sensitive. It can identify and quantitate the virus.
- Direct Fluorescent Antibody (DFA) – this test visualizes the presence of VZV in the cells taken from the patient’s skin lesion using a special microscope and labeled antibody. It is rapid, but less specific and sensitive than the VZV culture and DNA testing.
The choice of tests and samples collected depends on the patient, their symptoms, and on the doctor’s clinical findings.
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When is it ordered?
VZV antibody tests may be ordered to check immune status and/or to identify a recent infection. VZV culture or DNA tests may be ordered when a newborn or
immune-compromised person has been exposed to VZV and is ill with atypical and/or severe symptoms – to detect an active primary VZV infection in the baby or a primary or reactivated infection in the immune-compromised person.
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What does the test result mean?
Care must be taken when interpreting the results of VZV testing. The doctor evaluates the results in conjunction with clinical findings. It can sometimes be difficult to distinguish between a
latent and active VZV infection. This is possible for several reasons, including:
- A healthy person who has been infected with VZV will continue to harbor the virus after the symptoms disappear. The VZV can reactivate intermittently, often sub-clinically, shedding small amounts of virus into body fluids but not causing symptoms.
- An infant or immune-compromised person may not have a strong antibody response to the VZV infection – their IgM and IgG levels may be lower than expected even though they have an active case of VZV.
- The virus may not be present in sufficient number in the particular fluid or tissue tested to be detected.
Antibody detection
If both VZV IgG and IgM are present in a symptomatic person, then it is likely that she has either recently been exposed to VZV for the first time and has chickenpox or that the previous VZV infection has been reactivated and she has shingles.
If only IgM is present, then the infection may have been very recent. If a newborn has IgM antibodies, then he has a congenital VZV infection. If a person is symptomatic but has low or undetectable levels of IgG and/or IgM, it may mean that he either has a condition other than VZV or that his immune system is not responding normally – not producing a detectable level of VZV antibody.
Viral detection
If someone is symptomatic and the culture is positive for varicella zoster virus, then the person likely has an active VZV infection. If the culture is negative, then the person’s symptoms may be due to another cause or the VZV virus is not detectable in the sample tested.
If a test for VZV DNA is positive, then VZV is present. High levels of viral DNA tend to indicate an active infection. Low levels indicate a VZV infection but may not indicate a symptomatic condition. Negative results do not rule out VZV infection – the virus may be present in very low numbers or may not be present in the body sample tested.
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Is there anything else I should know?
There is now a
vaccine available for older adults that is intended to decrease the risk for having a re-activation of the virus that presents as shingles and decreases the severity of the disease if it does occur. It is not yet in widespread use and its ultimate effect on the incidence of shingles remains to be seen.
VZV may rarely cause encephalitis, a serious complication.
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