A viral test is done to find
infection-causing viruses. Viruses grow only in living cells. Viruses cause
disease by destroying or damaging the cells they infect, damaging the body's
immune system, changing the genetic material (DNA) of the cells they infect, or causing inflammation
that can damage an organ. Viruses cause many types of diseases, such as
human immunodeficiency virus (HIV),
measles, flu (influenza), and some types of
Chickenpox. This is caused by a form of the herpes
virus called varicella-zoster virus (VZV). A viral test may be done to see
whether a person has developed
immunity from having chickenpox or after getting the
Several types of tests may be used to check for
Antibody test.Antibodies are
substances made by the body's immune system to fight a specific viral
infection. The antibodies attach to a cell infected by the virus and cause the virus to be destroyed. This test looks for antibodies to a specific viral
infection. It is generally done on a blood sample. If the antibody is found,
this test can show whether a person was infected recently or in the
Viral antigen detection test. Viral
antigens develop on the surface of cells infected with
a specific virus. A viral antigen detection test is done on a sample of tissue
that might be infected. Specially tagged (with dye or a tracer) antibodies that
attach to those viral antigens are mixed with the sample. The tagged antibodies
can be seen by using a special light (or other method). If the tagged
antibodies are attached to the cells, the cells are infected with the
Viral culture. For a viral culture, a small sample
of tissue or fluid that may be infected is placed in a special cup along with
cells in which the virus can grow. If the virus grows in the culture, it will
cause changes in the cells that can be seen under a microscope. Viral cultures
may take several weeks to show results.
Viral DNA or RNA detection test. Using a sample of
tissue or blood or other fluid (such as spinal fluid), this type of test looks
for the genetic material (DNA or RNA) of a specific virus. This test can show
the exact virus causing an infection.
Different types of samples are used for a viral test,
including blood, urine, stool (feces), organ tissue, spinal fluid, and saliva.
The type of sample used for the test depends on the type of infection that may
Why It Is Done
A viral test is done to:
Find a viral infection that is causing
Check a person after exposure to a virus. For example, a
viral test may be done after a health professional is accidentally stuck with a
needle containing contaminated blood to see if he or she became infected with the virus.
viral infection in a potential blood donor to prevent the donation of infected
Find a viral infection in an organ to be
Test a pregnant woman who has a high risk of passing
a serious viral infection on to her baby.
Check if a person has
immunity to a specific virus.
How To Prepare
Preparations for a viral test depend
on the type of infection that may be present and the sample that will be
tested. Your health professional will give you any specific instructions before
How It Is Done
Samples can be collected in several
A blood sample can be taken from a vein in the
A tissue sample can be taken directly from the infection, such
as a throat swab or skin scraping.
A sample of stool, urine, or
nasal washings may be taken.
A sample of spinal fluid can be taken
lumbar puncture (spinal tap).
biopsy sample may be taken using a needle or other
How It Feels
The amount of discomfort or pain you
feel depends on the method used to collect a sample for the test. Generally, a
viral test does not cause pain or the pain goes away after the test.
Generally, the chance of problems from the test
depends on the method used to collect a sample for testing. Your doctor can talk to you about any specific risks of the test.
A viral test is done to find
It may take as little as 1 day or up
to several weeks to get test results.
The results of some viral
tests (antibody or
antigen tests) are reported in titers. A titer is a
measure of how much the sample can be diluted before the viral antibodies or
antigens can no longer be detected.
Depending on the virus, it can take weeks for antibodies
to develop after exposure to the virus. In these situations, test results may
be negative early in the course of the infection. This is called a
false-negative test result. Another blood sample may
need to be drawn later to check again for a viral infection. Antibody titers
that get higher over 3 weeks from the first sample to the second mean the
infection occurred recently.
Normal (results that do not show a viral infection are
No antibodies to the virus are
Viral antigen detection
No antigens made by the viral
infection are found.
Abnormal (results that show a viral infection are called
Antibodies to a virus are
found. But if you have a second antibody test and the results are not higher than the first test, this may mean the infection occurred in the past and is not a problem now.
Viral antigen detection
Viral antigens are
Changes occur in the culture
that show a viral infection.
Viral DNA or RNA detection
Viral DNA or RNA is
What Affects the Test
Reasons you may not be able to
have the test or why the results may not be helpful include taking antiviral
What To Think About
Sometimes positive antibody or antigen
detection test results are made by organisms other than the virus. This is
called cross-reactivity, which leads to a
false-positive test result. A test that shows a viral
infection may need to be confirmed by more tests.
an unborn baby (fetus) or newborn baby is tested for several kinds of
infections (including viral infections) all at the same time. This is called a TORCH test (for
toxoplasmosis, other infections,
herpes). The TORCH test shows whether a fetus or
newborn is likely to have any of these infections.
Depending on the
virus, it can take weeks for antibodies to develop after exposure to the virus.
In these cases, test results may be negative early in the course of the
infection. This is called a false-negative test result. Another blood sample
may need to be drawn later to check again for the viral infection. Antibody
titers that get higher over 3 weeks from the first sample to the second usually mean
the infection occurred recently.
Tests are available that can
identify many viruses from one sample of body fluid. For example, one test can
identify 12 different viruses that may be causing a lung
Spinal fluid is collected during a spinal tap (lumbar
puncture). To learn more, see the topic
How this information was developed to help you make better health decisions.