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:
- Herpes simplex.
- 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 chickenpox vaccine.
- Respiratory syncytial virus (RSV).
- Epstein-Barr virus.
- Cytomegalovirus (CMV).
- Genital warts (human papillomavirus, or HPV).
- Influenza (flu).
- Human immunodeficiency virus (HIV).
- BK virus.
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.
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 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 titres. A titre 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 titres 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 called negative):
No antibodies to the virus are found.
Viral antigen detection test:
No antigens made by the viral infection are found.
No viral infection is seen in the culture.
No viral DNA or RNA is found.
Abnormal (results that show a viral infection are called positive):
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.
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 titres 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.
Other Works Consulted
- 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.
Primary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine
Anne C. Poinier, MD - Internal Medicine
Adam Husney, MD - Family Medicine
Specialist Medical Reviewer W. David Colby IV, MSc, MD, FRCPC - Infectious Disease
Current as ofAugust 21, 2015
Current as of: August 21, 2015
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