Viral Tests

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:

• 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).
• Rotavirus.
• Hepatitis.
• Genital warts (human papillomavirus, or HPV).
• Influenza.

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 usually destroy the virus. 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. 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 be present.

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 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.

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.

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.

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 health professional 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 one day or up to several weeks to get test results.

The results of some viral tests (antibody or antigen tests) may be shown 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 seen.

A titer of 1 to 8 (1:8) means that antibodies or antigens can be found when 1 part of the blood sample is diluted by up to 8 parts of a salt solution (saline). A larger second number means there are more antibodies in the fluid sample. Therefore, a titer of 1 to 128 means more viral antibodies or antigens in the fluid sample than a titer of 1 to 32.

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.

Reasons you may not be able to have the test or why the results may not be helpful include taking some medicines, such as acyclovir (Zovirax), famciclovir (Famvir), or ganciclovir (Cytovene).

• 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 additional tests.
• Sometimes an unborn baby (fetus) or newborn baby is tested for several 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 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 the viral infection. Antibody titers that get higher over 3 weeks from the first sample to the second mean the infection occurred recently.
• Spinal fluid is collected during a spinal tap (lumbar puncture). For more information, see the medical test Lumbar Puncture.
• For more information on specific tests, see the following medical tests:
  • Human Immunodeficiency Virus (HIV) Test
  • Herpes Tests
  • Hepatitis A Virus Test
  • Hepatitis B Virus Test
  • Hepatitis C Virus Test
  • Human Papillomavirus (HPV) Test
  • Mononucleosis Test

Other Works Consulted

• Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.