There are three main types of HIV test.

The first type of test is the HIV antibody test. This test shows whether a person has been infected with HIV, the virus that causes AIDS. Information on this page concentrates mainly on HIV antibody testing. Antibody tests are also known as ELISA (Enzyme-Linked Immunosorbent Assay) tests.

The second type of test is an antigen test. Antigens are the substances found on a foreign body or germ that trigger the production of antibodies in the body. The antigen on HIV that most commonly provokes an antibody response is the protein P24. Early in the infection, P24 is produced in excess and can be detected in the blood serum by a commercial test (although as HIV becomes fully established in the body it will fade to undetectable levels). P24 antigen tests are sometimes used to screen donated blood, but they can also be used for testing for HIV in individuals, as they can detect HIV earlier than standard antibody tests. Some of the most modern HIV tests combine P24 and other antigen tests with standard antibody identification methods to enable earlier and more accurate HIV detection.

The third type of test is a DNA or RNA test. These types of tests detect the genetic material of HIV itself, and can identify HIV in the blood within a week of infection. DNA/RNA tests come in a number of forms. Babies born to HIV positive mothers may be tested using a type of DNA test called a PCR (Polymerase Chain Reaction). Blood supplies in developed countries are screened for HIV using an RNA test known as NAT (Nucleic Acid-amplification Testing). When a person already knows that she or he is infected with HIV, they may also have a viral load test to detect HIV genetic material and estimate the level of virus in the blood. DNA/RNA tests are rarely used to test for HIV in adults, as they are very expensive and more complicated to administer than a standard antibody or P24 test.

HIV testing

The standard HIV test looks for antibodies in a person's blood. When HIV (which is a virus) enters a person's body, special proteins are produced. These are called antibodies. Antibodies are the body's response to an infection. So if a person has antibodies to HIV in their blood, it means they have been infected with HIV. The only exception might be an HIV negative baby born to a positive mother. Babies retain their mother's antibodies for up to 18 months, so may test positive on an HIV antibody test, even if they are actually HIV negative. This is why babies born to positive mothers may receive a PCR test after birth.

Most people develop detectable HIV antibodies within 6 to 12 weeks of infection. In very rare cases, it can take up to 6 months. It is exceedingly rare for someone to take longer than 6 months to develop antibodies.

Getting tested earlier than 3 months may result in an unclear test result, as an infected person may not yet have developed antibodies to HIV. The time between infection and the development of antibodies is called the window period. During the window period people infected with HIV have no antibodies in their blood that can be detected by an HIV test. However, the person may already have high levels of HIV in their blood, sexual fluids or breast milk. Someone can transmit HIV to another person during the window period even though they do not test positive on an antibody test. So it is best to wait for at least 3 months after the last time you were at risk before taking the test, and in the meantime to abstain from sex. Some test centres may recommend testing again at 6 months, just to be extra sure.

It is also important that you are not exposed to further risk of getting infected with HIV during the window period. The test is only accurate if there are no other exposures between the time of possible exposure to HIV and testing.

If an individual's test is negative at six months and they have not had unprotected sex or shared needles again in the meantime, it means that they do not have HIV, and will not therefore go on to develop AIDS.

The only way to know for sure whether you are infected with HIV is to have an HIV antibody test. It is not possible to tell from any symptoms.