Viral tests

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 they 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.

Samples can be collected in several ways.

• A health professional uses a needle to take a blood sample, usually from 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.

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.

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 you've been exposed to the virus. In these cases, 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 taken 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.