Window period

In medicine, the window period for a test designed to detect a specific disease (particularly infectious disease) is the time between first infection and when the test can reliably detect that infection. In antibody-based testing, the window period is dependent on the time taken for seroconversion.

The window period is important to epidemiology and safe sex strategies, and in blood and organ donation, because during this time, an infected person or animal cannot be detected as infected but may still be able to infect others. For this reason, the most effective disease-prevention strategies combine testing with a waiting period longer than the test's window period.

Examples

• HIV AIDS: U.S. Government site www.AIDS.gov informs: "The timeframe between when you are exposed to HIV to the time you test positive for HIV antibodies can be up to 3-6 months. This period of time is called a “window period” for HIV testing. On average, you may need to wait 2 to 8 weeks from the time of possible exposure to get an accurate test result, because it takes at least that long for the immune system to develop enough HIV antibodies to be detectable."

Related Wikipedia article informs: "Antibody tests may give false negative (no antibodies were detected despite HIV being present) results during the window period, an interval of three weeks to six months between the time of HIV infection and the production of measurable antibodies to HIV seroconversion. Most people develop detectable antibodies approximately 30 days after infection, although some seroconvert later. The vast majority of people (97%) have detectable antibodies by three months after HIV infection; a six-month window is extremely rare with modern antibody testing. During the window period, an infected person can transmit HIV to others although their HIV infection may not be detectable with an antibody test. Antiretroviral therapy during the window period can delay the formation of antibodies and extend the window period beyond 12 months. This was not the case with patients that underwent treatment with post exposure prophylaxis (PEP). Those patients must take ELISA tests at various intervals after the usual 28 day course of treatment, sometimes extending outside of the conservative window period of 6 months. Antibody tests may also yield false negative results in patients with X-linked agammaglobulinemia; other diagnostic tests should be used in such patients."

• Hepatitis B: During the window period (or equivalence zone) of Hepatitis B, both serological markers HBsAg (Hepatitis B surface antigen) and Anti-HBs (antibody against HBsAg) are negative (which is due to the fact that, although there are Anti-HBs antibodies present, they are actively bound to the HBsAg). Other serological markers, IgM (antibody) against HBc can be positive at this point.

See also

• Incubation period, the time between infection and the appearance of symptoms