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Viral Tests

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include taking antiviral medicines.

What To Think About

  • Sometimes positive antibody or antigen detection test results are made by organisms other than the virus. This is called cross-reactivity, which leads to a false-positive test result. A test that shows a viral infection may need to be confirmed by more tests.
  • Sometimes an unborn baby (fetus) or newborn baby is tested for several kinds of infections (including viral infections) all at the same time. This is called a TORCH test (for toxoplasmosis, other infections, rubella, cytomegalovirus, and herpes). The TORCH test shows 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 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 drawn later to check again for the viral infection. Antibody titers that get higher over 3 weeks from the first sample to the second usually mean the infection occurred recently.
  • Tests are available that can identify many viruses from one sample of body fluid. For example, one test can identify 12 different viruses that may be causing a lung infection.
  • Spinal fluid is collected during a spinal tap (lumbar puncture). To learn more, see the topic Lumbar Puncture.

To learn more about a specific test, see:

Other Works Consulted

  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.

ByHealthwise Staff
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerW. David Colby IV, MSc, MD, FRCPC - Infectious Disease
Last RevisedDecember 30, 2011

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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