HIV can be diagnosed in several different ways. The most common screening test for HIV is called an ELISA. This test checks for antibodies (the body's response to infection) against HIV. This test can be run on either blood or saliva, but tends to be more reliable on a blood sample. The purpose of this test is to screen and not miss any possibly infected person, but there will be occasional false positive tests. Since this can occur, any positive ELISA test will need to be confirmed by a more complex test called a Western Blot. This test takes a much closer look at the antibodies to confirm the diagnosis. This test may take 3-7 days to run and to confirm the diagnosis.

In very small babies, a viral load test that actually measures the amount of virus rather than an antibody response may be more accurate and may be obtained to confirm the diagnosis. A mother will pass her HIV antibodies to the baby. It can take months for the mother's antibodies to clear in the baby's system. The viral load test is done in order to determine if the virus itself is present in the baby's system. The viral load can show quickly if the virus is present in the baby. If virus is detected in the baby, the baby is infected. This results in quicker diagnosis of the baby and allows for early treatment. For all other patients, the viral load test is not used to make a diagnosis since it is expensive and is used instead to monitor the disease in known HIV positive patients.