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p24 Antigen

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Also known as: p24 capsid or core antigen; p24 antigen capture assay; p24 protein; HIV antigen; HIV antigen/antibody combination testing
Formal name: p24 Antigen

The Test Sample

What is being tested?

HIV is the virus that causes AIDS (acquired immunodeficiency syndrome). The p24 antigen is an HIV viral protein and the level in the blood is typically elevated early in the course of infection, before antibodies to the virus have been produced. This test detects the p24 antigen to screen for early HIV infection. Typically, it is performed as part of a combination test (HIV antibody/p24 antigen). Less commonly, it is performed as an individual test.

When HIV enters the body, such as through contact with the blood or body fluids of an infected individual or exposure to a contaminated needle, the virus begins to replicate itself, producing a large number of copies. During the first few weeks of infection, the amount of virus (viral load) and the p24 antigen level in the blood can be quite high. The affected person typically experiences flu-like symptoms that resolve as the body's immune system begins to produce antibodies directed against HIV. Both virus and p24 antigen levels decrease in the blood as the initial infection resolves and the level of HIV antibody increases. In untreated persons, the infection then simmers for a decade or more, causing few symptoms but slowly degrading the immune system.

With the diminishing immune response, eventually symptoms of AIDS emerge and begin to progressively worsen. HIV viral loads and viral protein levels increase. The weakened immune system leaves the affected person vulnerable to debilitating infections. Treatments for AIDS suppress the amount of HIV virus present in the blood and limit its ability to replicate, thereby slowing damage to the immune system. Increases and decreases in HIV are mirrored with increases and decreases in p24 antigen.

The use of the p24 antigen test as an individual test has declined somewhat. However, combination tests that detect both p24 antigen and HIV antibody have been developed and are used to increase the likelihood of detecting HIV infection soon after exposure to the HIV virus. Still, the p24 test as an individual test may be used in areas where resources are limited and where molecular tests are not as widely available.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.