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The HIV antigen is a protein found in the core structure of the human immunodeficiency virus (HIV), the organism that causes AIDS. This protein is known as p24, and its presence is used as a test for infection by the virus. In the 1990s, this test was used in the United States to screen blood before it entered the blood supply. Now more specific tests are available for most HIV testing. The p24 antigen test is currently used in the first few weeks after a person may have been infected, before they have had a chance to build up antibodies to the virus.
Antigens are molecules that react with an antibody. Each individual produces millions of different antibodies, each specific for different molecules. Such antibodies can be used to test for the presence of infectious organisms. Generally, such tests are designed to react to a protein on the surface of the organism. In this case, the p24 protein is found outside of the virus, as a soluble protein after the virus has replicated.
The classic test to detect an HIV infection is an ELISA test to detect a person’s antibodies to the HIV virus. If a person tests positive for HIV with an ELISA test, their blood is subjected to a more rigorous test known as a Western blot. If both tests are positive, the person is deemed infected.
The p24 protein is produced during the initial phases of infection, then declines to undetectable levels as it binds to the HIV antibodies. If a person is newly infected and has not yet developed HIV antibodies, the classic HIV antibody test will be ineffective at diagnosing their infection. In these cases, an HIV antigen test is utilized to measure p24 levels.
A modified ELISA test is used with antibodies to the p24 protein. A negative HIV antigen result is unclear. It could mean that the person is uninfected, or that he or she has the virus but is producing a low level of p24. A positive result is a definitive sign of infection.
This test can be used when there is good reason to suspect infection may have occurred, such as with occupational exposure or cases of rape. A blood antigen test for the p24 protein used to be the state-of-the-art test for HIV testing. The HIV antigen test is not used as widely as it was in the past. There are now tests that can directly measure the RNA of the virus, and more accurately provide a diagnosis early during the course of infection.