The p24 antigen test detects a protein found in the core structure of HIV and is used in the first few weeks after infection before antibodies develop. It is less commonly used now as more specific tests are available.
HIV antigen is a protein found in the core structure of 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 bloodstream. More specific tests are now available for most HIV tests. 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 develop antibodies to the virus.
Antigens are molecules that react with an antibody. Each individual produces millions of different antibodies, each specific for different molecules. These antibodies can be used to test for the presence of infectious organisms. Typically, these tests are designed to react to a protein on the surface of the body. In this case, the p24 protein is found outside the virus, as a soluble protein after the virus has replicated.
The classic test for detecting 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 stringent test known as a Western blot. If both tests are positive, the person is considered infected.
The p24 protein is produced during the initial stages of the infection, then drops to undetectable levels as it binds to HIV antibodies. If a person has just been infected and has not yet developed HIV antibodies, the classic HIV antibody test will be ineffective in diagnosing the infection. In these cases, an HIV antigen test is used to measure p24 levels.
A modified ELISA test with antibodies against the p24 protein is used. A negative HIV antigen result is unclear. It could mean that the person is not infected or that they have the virus but are 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 that an infection has occurred, such as in occupational exposure or rape cases. A blood antigen test for p24 protein was the state-of-the-art test for HIV testing. HIV antigen testing is not as widely used as it used to be. There are now tests that can measure virus RNA directly and provide a more accurate diagnosis earlier in the course of infection.
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