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Why a negative HIV test does not rule out infection?

Q: The ELISA test (screening test) and the Western blot (confirmatory test) are blood tests used to detect HIV antibodies. A negative test does not guarantee that a person is not infected. The above statment is from your web site . my question is that Why A negative test does not guarantee that a person is not infected ?

A:The usual tests for HIV testing look for the antibodies produced against the virus by the body. Since the immune system takes some time to do this, in the initial couple of months the test for these may be negative. This phase when the routine tests for HIV testing are negative is known as the `window phase`. There are diagnostic modalities (molecular methods - PCR, NASBA, etc) capable of detecting the infection in the window phase (in special cases) but these are quite expensive and are not used as a routine in all cases unless the screening tests are done first.

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