Viral Test

Posted On Thursday, June 14, 2007   


"A viral test is done to find infection-causing viruses. Viruses are one of the smallest organisms and, unlike bacteria, they 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, 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 AIDS, cold sores, chickenpox, measles, flu (influenza), and some types of cancer.
 
A viral test is most commonly done to help detect:
 
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.
A viral test is also commonly done to detect:
 
Cytomegalovirus (CMV).
Rotavirus.
Hepatitis.
Genital warts (human papillomavirus, or HPV).
Influenza.
Several types of tests may be used to detect 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 a viral test, 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.
 
Why It Is Done
  • A viral test is done to:
  • Find and identify a viral infection that is causing symptoms.
  • Check a person after exposure to a virus. For instance, viral testing may be done after a health professional is accidentally stuck with a needle.
  • 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 child.
  • 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.
How It Is Done
  • Samples can be collected in several ways.
  • A blood sample can be collected from a vein in the arm.
  • A tissue sample can be collected directly from the location of infection, such as a throat swab or skin scraping.
  • A sample of stool, urine, vaginal discharge, or saliva may be collected.
  • A sample of spinal fluid can be collected through a lumbar puncture (spinal tap). For more information, see the medical test Lumbar Puncture.
  • A biopsy sample may be collected using a needle or other instrument.
How It Feels
  • The amount of discomfort or pain you feel depends on the method used to collect a sample for the test.
Risks
  • The risk of the test depends on the method used to collect a sample for testing.
Results
  • A viral test is done to detect 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 expressed 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.
  • 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.
  • 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 evaluate the possible presence of a viral infection. Antibody titers that increase over 3 weeks, from the first sample to the second, indicate a recent infection.
  • Viral test Normal (results that do not detect a viral infection are called negative):
  • Antibody test:
  • No antibodies to the virus are detected.
  • No rise in antibody titers indicates an infection occurred in the past versus a recent infection.
  • Viral antigen detection test:
  • No antigens produced by a viral infection are detected.
  • Viral culture:
  • No evidence of viral infection is seen in the culture.
  • Viral DNA or RNA detection test:
  • No viral DNA or RNA is detected.
  • Abnormal (results that indicate a viral infection are called positive):
  • Antibody test:
  • Significant antibodies to a virus are detected.
  • Viral antigen detection test:
  • Viral antigens are detected.
  • Viral culture:
  • Changes occur in the cells of the culture that indicate a viral infection.
  • Viral DNA or RNA detection test:
  • Viral DNA or RNA is detected.
What Affects the Test
  • Factors that can interfere with your test and the accuracy of the results include:
  • Medications. These include acyclovir (Zovirax), famciclovir (Famvir), or ganciclovir (Cytovene).
  • Contamination or inadequate refrigeration of the sample used for a viral test.
What To Think About
  • Sometimes positive antibody or antigen detection test results are produced by organisms other than the indicated virus. This is called cross-reactivity, which leads to a false-positive test result. Therefore, test results that indicate the presence of a viral infection may need to be confirmed by additional tests.
  • Sometimes a fetus or newborn baby is tested for several infections, including 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 determines 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 evaluate the possible presence of a viral infection. Antibody titers that increase over 3 weeks, from the first sample to the second, indicate a recent infection.
  • For more information on specific tests, see the following medical tests:
  • Human Immunodeficiency Virus Test
  • Herpes Tests
  • Hepatitis A Virus Test
  • Hepatitis B Virus Test
  • Hepatitis C Virus Test
  • Human Papillomavirus (HPV) Test
  • Mononucleosis Test



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