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HIV Antibody and HIV Antigen (p24)

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Also known as: HIV Screening Tests; AIDS Test; AIDS Screen; HIV Serology; p24 Antigen
Formal name: HIV-1 and HIV-2 Antibody and Antigen Evaluation

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The Test Sample

What is being tested?

Human immunodeficiency virus (HIV) is the cause of AIDS (acquired immunodeficiency syndrome). HIV screening tests detect the HIV antigen (p24) and/or HIV antibodies produced in response to an HIV infection in the blood. Some tests detect HIV antibody in oral fluid.

When a person becomes infected with HIV, through exposure to the blood or body fluids of an infected individual or a contaminated needle, for example, 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.

About 2-8 weeks after exposure to the virus, the immune system responds by producing antibodies directed against the virus that can be detected in the blood. As the initial infection resolves and the level of HIV antibody increases, both virus and p24 antigen levels decrease in the blood.

An HIV infection may initially cause no symptoms or cause flu-like symptoms that resolve after a week or two. The only way to determine whether a person has been infected is through HIV testing.

If HIV is not detected early and treated, it may become a simmering infection that may cause few symptoms for a decade or more. If the infection is still not treated, eventually symptoms of AIDS emerge and begin to progressively worsen. Over time and without treatment, HIV destroys the immune system and leaves a person's body vulnerable to debilitating infections. (You can read more about this in the article on HIV Infection and AIDS.)

Detecting and diagnosing HIV early in the course of infection is important because:

  • It allows for early treatment that slows progression to AIDS.
  • An individual can learn of their status and modify behavior so as to prevent the spread of disease.
  • A pregnant woman can undergo treatment that would help prevent passing the disease to her child.

There are two types of HIV, 1 and 2. HIV-1 is the most common type found in the United States, while HIV-2 has a higher prevalence in parts of Africa.

A few different testing options are available for HIV screening:

  • Combination HIV antibody and HIV antigen test—the recommended screening test for HIV; it is available only as a blood test.
  • HIV antibody testing—all HIV antibody tests used in the U.S. detect HIV-1 and some tests have been developed that can also detect HIV-2. These tests are available as blood tests or tests of oral fluid.
  • p24 antigen testing—this is used alone without the antibody test only in rare cases when there is a question about interference with an HIV antibody test.

Regardless of the type of screening test used, a positive result requires follow up with supplemental testing to establish a diagnosis of HIV. (For more details, see "The Test" section.)

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm or from a fingerstick. There also are tests available that can be performed on oral samples. An oral sample is obtained by using a special small, spatula-like device with a flat pad on the end. The flat pad is placed above the teeth against the outer gum and is swabbed completely once around the outer part of the upper and lower gums.

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.