Q. What is the window phase of an infection? How is this concept important for the test of HIV infection in blood banks?

The primary immune reply of the body facing any infection isn't immediate. The window phase is the time from the infection until the formation of detectable specific antibodies against the infective agent. In this time, immunochemical tests that indirectly search infections, like the ELISA and the Western-blot for HIV, give a negative result even if the person is actually infected by the agent. In the case of HIV, the window phase can last about 2 weeks to 3 months.

The window phase is a big trouble for blood banks that perform only immunochemical tests on the donated blood. This because, why in some countries the blood donors are submitted to a series of questions regarding their prior behavior, mainly sexual and drug use related, in the months preceding the donation and also to voluntary confidential self exclusion, in which they declare whether their blood may be used or should be excluded. Instead of facing this dangerous problem, modern blood banks nowadays use the PCR (polymerase chain reaction) for HIV, a test that is not distorted by the window phase since it is not antibody-based. Using specific primers, the PCR detects the presence of DNA generated by reverse transcription of the HIV RNA.