After someone is infected with HIV, it can take some time for their body to develop antibodies to the virus.  It is these antibodies that a test looks for.  This is known as the ‘window period’.   This means that if you were infected shortly before you have a test, it might not show up.  Most of the HIV tests used are very sensitive and will detect HIV one month after someone is exposed, so a negative test is highly likely to be accurate.  However, it is recommended that an additional HIV test is offered at 12 weeks to everyone who tests to make sure that they were not in the window period and are not infected.

If you are asked to have a follow-up test, you should take the following precautions while waiting for your second result: