HIV testing

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.

Blood supplies in most developed countries are screened for HIV using an RNA PCR test, which can produce positive results several days before a DNA test.

The third type of test is a PCR test (Polymerase Chain Reaction test). The whole process of extracting genetic material and testing it with a PCR test is referred to as Nucleic Acid-amplification Testing or 'NAT'. PCR tests detect the genetic material of HIV itself, and can identify HIV in the blood within two or three weeks of infection.

PCR tests come in two forms: DNA PCR and RNA PCR. Babies born to HIV positive mothers are usually tested using a DNA PCR because they retain their mother's antibodies for several months, making an antibody test inaccurate. Blood supplies in most developed countries are screened for HIV using an RNA PCR test, which can produce positive results several days before a DNA test. 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. This can be performed using either an RNA or DNA PCR test. PCR tests are not often used to test for HIV in adults, as they are very expensive and more complicated to administer than a standard antibody or P24 test. However they may be offered in special circumstances, or by private clinics where patients are willing to pay.