Viruses are the smallest infection-causing organisms. Unlike bacteria, viruses multiply only within living cells. They can cause disease by destroying or disabling most of the cells they infect, by damaging the body's immune system, by changing the genetic material (DNA) of the cells they infect, or by causing inflammation that can damage an organ. Viruses cause many types of diseases, such as AIDS, cold sores, chickenpox, measles, flu (influenza), and some types of cancer.

Viral testing is most commonly done to help reveal:

• Human immunodeficiency virus.
• Herpes simplex.
• Chickenpox, which is caused by a form of the herpes virus called varicella-zoster virus (VZV). Viral testing may be done to evaluate whether a person has developed immunity from a previous infection or after receiving the chickenpox vaccine.
• Respiratory syncytial virus (RSV).
• Epstein-Barr virus.

Viral testing is also commonly done to find out:

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

Several types of tests may be used to reveal 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 a virus and usually cause it to be destroyed. An antibody test detects the antibodies formed to fight a specific viral infection. It is usually done on a blood sample. If the type of antibody is identified, this test can determine 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 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 technique). 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 container 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 complete.
• Viral DNA or RNA detection test. Using a sample of tissue or blood or other fluid (such as spinal fluid), this type of test detects the genetic material (DNA or RNA) of a specific virus. This test can identify the exact virus causing an infection.

Except for the antibody test, these tests usually cannot determine whether a person was infected recently or in the past.

Different types of samples are used for viral testing, including blood, urine, stool (feces), organ tissue, spinal fluid, and saliva. The type of sample used for testing depends on the type of infection that may be present.

Viral Testing
Results

It may take only one day or up to several weeks to get test results.

The results of some viral tests (antibody or antigen tests) may be expressed in titers. A titer is a measure of how much the sample can be diluted before the viral antibodies or antigens cannot be detected any longer.

A titer of 1 to 8 (1:8) means that antibodies or antigens can be detected 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 indicates more viral antibodies or antigens in the fluid sample than a titer of 1 to 32.