Icon responsive menu red

  Back Blood-tests Explained


Chickenpox


At a Glance

Why Get Tested?

To diagnose, as necessary, a current, recent, or past case of chickenpox or shingles; to demonstrate immunity to the varicella zoster virus (VZV) or the potential for reactivating a VZV infection prior to receiving immunosuppressive drugs

When to Get Tested?

When you have atypical and/or severe symptoms and your doctor wants to distinguish between a VZV infection and another cause; when a doctor wants to check whether or not you are immune to VZV; sometimes prior to an organ transplant or when a child, pregnant woman, or an immune-compromised person has been exposed to someone with chickenpox

Sample Required?

A blood sample drawn from a vein in your arm for VZV antibody testing; to detect the virus itself, a sample of fluid from a blister (vesicle), blood, cerebrospinal fluid, or other body fluid or tissue

Test Preparation Needed?

None

The Test Sample

What is being tested?

Chickenpox and shingles are caused by an infection with the varicella zoster virus (VZV), a member of the herpes virus family. Varicella zoster virus tests detect either antibodies produced by the immune system in response to a VZV infection or detect the virus itself.

Tests for chickenpox and shingles may be performed to detect and diagnose a current or past infection with VZV. Most often, testing is not necessary to diagnosis an active infection because it can be made from clinical signs and symptoms, but in some people with atypical skin lesions, a diagnostic test helps to confirm the infection. In some people, especially organ transplant recipients and pregnant women, the tests may be used to diagnose a current infection or to determine whether or not they have developed immunity from prior infection or by vaccination.

Before the introduction and widespread use of a chickenpox vaccine in 1995, nearly everyone in the United States became infected by VZV by the time they were an adult. While VZV is present in its latent form in many adults who were infected as children, according to the Centers for Disease Control and Prevention, the incidence of new cases of chickenpox has declined significantly. Two doses of the vaccine are about 98% effective in preventing the infection, and those who do become infected usually have milder symptoms.

Varicella zoster virus can cause chickenpox in the young and in adults who have not been vaccinated or previously exposed. The primary infection is highly contagious, passing from person to person through coughing or sneezing or touching fluid from blisters. In a primary infection, an itchy rash emerges about two weeks after exposure to the virus, followed by the formation of pimple-like papules that become small, fluid-filled blisters (vesicles). The vesicles break, form a crust, and then heal. This process occurs in two or three waves or "crops" of several hundred vesicles over a few days.

Once the initial infection has resolved, the virus becomes latent, persisting in sensory nerve cells. The person develops  antibodies during the infection that usually prevent them from getting chickenpox again. However, later in life and in those with weakened immune systems, the virus can reactivate, migrating down the nerve cells to the skin, causing shingles (also known as herpes zoster). Symptoms of shingles include a mild to intense burning or itching pain in a band of skin at the waist, the face, or another location. It is usually in one place on one side of the body but can also occur in multiple locations. Several days after the pain, itching, or tingling begins, a rash, with or without vesicles, forms in the same location. In most people, the rash and pain subside within a few weeks, and the virus again becomes latent. A few may have pain that lingers for several months.

A shingles vaccine is now available for older adults that lowers the risk for having a reactivation of the virus that presents as shingles and lessens the severity of the symptoms if it does occur.In 2006, the Advisory Committee on Immunization Practices (ACIP) began recommending a shingles vaccination for all adults aged 60 years and older. However, the vaccine is not recommended for those who have weakened immune systems.

Most cases of chickenpox and shingles resolve without complications. In people with weakened immune systems, such as those with HIV/AIDS or organ transplant recipients, it can be more severe and long-lasting. In some cases, it may not become latent and may spread to the central nervous system.

In pregnant women, the effects of exposure to VZV on a developing baby or newborn depend on when it occurs and on whether or not the mother has been previously exposed. In the first 20 to 30 weeks of pregnancy, a primary VZV infection may, rarely, cause congenital abnormalities in the unborn baby. If the infection occurs one to three weeks before delivery, the baby may be born with or acquire chickenpox after birth, although the baby may be partially protected by the mother's antibodies. If a newborn is exposed to VZV at birth and does not have maternal antibody protection, then the VZV infection can be fatal.

How is the sample collected for testing?

The sample required depends on whether testing is being done to determine the presence of antibodies or to detect the virus itself and on the health status of the person. Antibody testing requires a blood sample drawn from a vein in the arm. Viral detection may be done on a variety of samples, including a sample of vesicle fluid, blood, cerebrospinal fluid, other body fluid, or tissue.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it used?

These tests are not routinely used to diagnose active cases of chickenpox and shingles, which are caused by the varicella zoster virus (VZV). These conditions are usually diagnosed based upon a person's signs and symptoms. 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 people prior to organ transplantation, and in those with HIV/AIDS. Testing may be used to:

  • Determine if someone has been previously exposed to VZV either through past infection or vaccination and has developed immunity to the disease
  • Distinguish between an active or prior infection
  • 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 someone is exposed to VZV, their immune system responds by producing antibodies to the virus. Two classes 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 and 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 DNA testing – performed to detect VZV genetic material in a person's 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 a person'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.
  • VZV culture – culture is not very reliable for VZV and can lead to false-negative results.

The choice of tests and samples collected depends on the person, their symptoms, and on the doctor's clinical findings.

When is it ordered?

VZV antibody tests may be ordered when a doctor wants to check a person's immune status, especially someone who is at high risk such as organ transplant recipients and pregnant women, and/or 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.

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 the person has either recently been exposed to VZV for the first time and has chickenpox or that the previous VZV infection has been reactivated and the person has shingles.

If only IgM is present, then the infection may have been very recent. If a newborn has IgM antibodies, then the baby 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 the person either has a condition other than VZV or that the person's 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.

Is there anything else I should know?

VZV may rarely cause encephalitis, a serious complication.



Disclaimer:

MedSquirrel is a shared knowledge, collective intelligence digital platform developed to share medical knowledge between doctors and patients. If you are a healthcare practitioner, we invite you to criticise, contribute or help improve our content. We support collaboration among all members of the healthcare profession since we strive for the provision of world-class, peer-reviewed, accurate and transparent medical information.

MedSquirrel should not be used for diagnosis, treatment or prescription. Always refer any questions about diagnosis, treatment or prescription to your Doctor.