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Viral Tests

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.

Risks

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.

Results

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 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.

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.

Viral test

Normal (results that do not show a viral infection are called negative):

Antibody test:

No antibodies to the virus are found.

Viral antigen detection test:

No antigens made by the viral infection are found.

Viral culture:

No viral infection is seen in the culture.

Viral DNA or RNA detection test:

No viral DNA or RNA is found.

Abnormal (results that show a viral infection are called positive):

Antibody test:

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.

Viral culture:

Changes occur in the culture that show a viral infection.

Viral DNA or RNA detection test:

Viral DNA or RNA is found.

 

WebMD Medical Reference from Healthwise

Last Updated: December 30, 2011
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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