Test Overview

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),
cold sores,
chickenpox,
measles, flu (influenza), and some types of
cancer.

Viral tests may be done for viruses such as:

Several types of tests may be used to check for
viruses:

  • 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
    past.
  • 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
    virus.
  • Viral culture. This is a test to find a virus that can cause an infection. A sample of body fluid or tissue is added to certain cells used to grow a virus. If no virus infects the cells, the culture is negative. If a virus that can cause infection infects the cells, the culture is positive. A viral culture
    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
be present.

Why It Is Done

A viral test is done to:

  • Find a viral infection that is causing
    symptoms.
  • 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.
  • Find a
    viral infection in a potential blood donor to prevent the donation of infected
    blood.
  • Find a viral infection in an organ to be
    transplanted.
  • 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
your test.

How It Is Done

Samples can be collected in several
ways.

  • A blood sample can be taken from a vein in the
    arm.
  • 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
    through a
    lumbar puncture (spinal tap).
  • A
    biopsy sample may be taken using a needle or other
    tool.

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.

Risks

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.

Results

A viral test is done to find
infection-causing viruses.

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.

Viral test

Normal (results that do not show a viral infection are
called negative):

Antibody test:

No antibodies to the virus are
found.

Viral antigen detection
test:

No antigens made by the viral
infection are found.

Viral culture:

No viral infection is seen in
the culture.

Viral
DNA or
RNA detection test:

No viral DNA or RNA is
found.

Abnormal (results that show a viral infection are called
positive):

Antibody test:

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
test:

Viral antigens are
found.

Viral culture:

Changes occur in the culture
that show a viral infection.

Viral DNA or RNA detection
test:

Viral DNA or RNA is
found.

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
medicines.

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.
  • Sometimes
    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,
    rubella,
    cytomegalovirus, and
    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
    infection.
  • Spinal fluid is collected during a spinal tap (lumbar
    puncture). To learn more, see the topic
    Lumbar Puncture.

References

Other Works Consulted

  • Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.

Credits

ByHealthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD – Internal Medicine
Adam Husney, MD – Family Medicine
Specialist Medical Reviewer Martin J. Gabica, MD – Family Medicine

Current as ofOctober 9, 2017