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.

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 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.