It is possible to test for chickenpox, also known as varicella, through laboratory methods. While healthcare providers often diagnose chickenpox based on its characteristic rash and symptoms, specific tests can confirm the presence of the varicella-zoster virus (VZV) or detect evidence of past infection or immunity. These laboratory tests serve as valuable tools, particularly in situations where a clear clinical diagnosis is not immediately apparent.
Types of Chickenpox Tests
Several laboratory methods are available to detect the varicella-zoster virus or antibodies against it. Polymerase Chain Reaction (PCR) testing is a highly sensitive and specific method for confirming an active infection. It detects viral DNA from samples like fluid from skin lesions or scabs, providing rapid results.
The Direct Fluorescent Antibody (DFA) test identifies viral antigens from skin scrapings. Though rapid, DFA is less sensitive than PCR. These tests are performed on samples collected directly from the rash.
Serology, or antibody testing, involves blood tests for specific antibodies produced in response to VZV. Immunoglobulin M (IgM) antibodies indicate a recent or current infection, appearing within one to two weeks after exposure. Immunoglobulin G (IgG) antibodies signify a past infection or successful vaccination, suggesting long-term immunity.
When Testing is Recommended
Testing for chickenpox is not always necessary, as a typical case can be diagnosed by a healthcare provider’s visual examination of the rash. However, laboratory confirmation is important in specific scenarios where the diagnosis is uncertain or health risks are elevated. This includes atypical presentations of the rash, especially in vaccinated individuals who may develop a milder form of the disease with fewer or non-vesicular lesions, making identification challenging.
Testing is recommended for individuals with weakened immune systems, as symptoms may be atypical or infection more severe. Confirmation allows for appropriate and timely management. Pregnant women without known immunity to chickenpox may undergo testing, particularly if exposed to the virus, due to risks to mother and fetus.
Newborns and infants may be tested to confirm congenital or perinatal VZV infection, which can have serious health implications. Testing also helps differentiate chickenpox from other conditions with similar rashes, such as herpes simplex, enterovirus infections, impetigo, measles, or insect bites. In public health, testing aids outbreak investigations or verifies immunity for professions like healthcare workers, or for school entry.
Understanding Test Results
Interpreting chickenpox test results requires a healthcare professional’s assessment in conjunction with the patient’s symptoms and medical history. For PCR or DFA tests, a positive result indicates the active presence of the varicella-zoster virus, confirming a current infection. A negative result suggests the virus was not detected in the sample, but it does not rule out a very early or mild infection with low viral load.
For serological antibody tests, the interpretation varies depending on the type of antibody found. A positive IgM result suggests a recent or ongoing chickenpox infection. A positive IgG result indicates prior VZV infection or vaccination, signifying immunity.
If both IgM and IgG results are negative, it means the individual has no current infection, prior exposure, or immunity, making them susceptible. An equivocal result may occur, suggesting retesting after a period. Healthcare providers integrate these results with clinical observations to determine the appropriate course of action.