It s important to have an HIV test if you think you have been at risk of HIV infection.There are effective treatments available that slow the growth of the virus. Being diagnosed soon after you are infected can help you get the best out of the treatments. The test for HIV is a blood test. It measures antibodies to HIV that are the body s immune response to the HIV. The test used most commonly for diagnosing infection with HIV is referred to as an ELISA. If the ELISA finds HIV antibodies, the results must be confirmed, typically by a test called a Western blot. HIV antibody tests remain the best method for diagnosing HIV infection. Antibodies to HIV typically develop within several weeks(3 months) of infection. During this interval, patients have virus in their body but will test negative by the standard antibody test, the so called window period. In this setting, the diagnosis can be made if a test is used that actually detects the presence of virus in the blood rather than the antibodies, such as tests for HIV RNA or p24 antigen. Recently, a new test has been approved that measures both HIV antibodies and p24 antigen, shrinking the duration of the window period from infection to diagnosis.