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 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.

As an example, since window periods for HIV testing are up to approximately 6 months (occasionally longer), somebody who had received a tattoo putting them at risk of infection might be required to wait 12 months before donating blood.

HIV/AIDS | Sexually-transmitted diseases