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p24 Antigen

Also known as: p24 capsid or core antigen; p24 antigen capture assay
Formal name: p24 antigen
Related tests: HIV antibody; HIV viral load; CD4 and CD8; HIV genotypic resistance testing; HIV phenotypic resistance testing

At a Glance

Why Get Tested?

To check for infection with HIV after a recent exposure

When to Get Tested?

If you have been recently exposed to HIV

Sample Required?

A blood sample drawn from a vein in your arm or by a fingerstick

Test Preparation Needed?

None

The Test Sample

What is being tested?

The p24 antigen test detects actual HIV viral protein in blood. The test is generally positive from about one week to 3-4 weeks after infection with HIV. The p24 protein cannot be detected until about a week after infection with HIV because it generally takes that long for the virus to become established and multiply to sufficient numbers that they can be detected. About 2-8 weeks after initial exposure, antibodies are produced in response to HIV infection. Once antibodies are produced, the results of the p24 test will usually be negative although the person may be infected with HIV. The antibodies bind to the p24 protein, causing the p24 antigen to no longer be detected in the blood. At that point, however, the HIV antibody test that is most often used for routine screening will be positive. Later in the course of HIV, p24 protein levels again become detectable if the disease is untreated.

The use of this test has declined somewhat, especially with the increase in use of molecular tests to detect early infections (see Viral Load) and to screen blood and blood products. However, it is often used in areas where resources are limited and where molecular tests are not as widely available.

Sometimes this test may be combined with a test for HIV antibodies to increase the likelihood of detecting HIV infection in the early stages.

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.

The Test

How is it used?

p24 antigen testing may be used to help diagnose early HIV infection. Levels of p24 antigen increase significantly at about one to three weeks after initial infection. It is during this time frame before HIV antibody is produced when the p24 test is useful in helping to diagnose infection. About 2-8 weeks after exposure, antibodies to HIV are produced and remain detectable in response to the infection, making the HIV antibody test the most useful assay to diagnose an infection.

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When is it ordered?

If the HIV antibody test is negative and you have had a recent exposure to HIV, a p24 test may be ordered to detect the infection. However, this test is not ordered as frequently as it once was since tests that can detect HIV RNA early in infections (HIV viral load) have become more widely available. Tests for HIV antibody are the most commonly ordered screening test.

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What does the test result mean?

A positive result means that you are infected with HIV.

A negative result may mean that you are not infected with HIV or that the level of p24 is below the detectable limits of the test. If you suspect that you have been exposed or if you are at an increased risk, repeat testing or screening with a different test, such as the HIV antibody test, is recommended.

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Is there anything else I should know?

The p24 test is one of the earlier tests developed to detect HIV infection. Because p24 is only detectable during a short window of time, its utility is limited. However, this test can still be used when other tests are unavailable.

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Common Questions

1.  Are there methods for testing for HIV other than a blood test?

Yes. Although the blood test for HIV antibody is the most commonly used test for HIV, new methods that use either saliva or urine are also available in some locations.

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2.  What are the symptoms of HIV infection?

Symptoms of acute HIV infection may mimic the flu or other systemic illnesses. The only reliable way to tell if you are infected with HIV is to get tested. Many people who have HIV do not experience symptoms related to HIV for many years. For more information on symptoms of HIV infection, visit the CDC’s web site.

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

(Dec. 1, 2007) Mayo Clinic.com. HIV Testing, What tests and when to get tested. Available online at http://www.mayoclinic.com/health/hiv-testing/ID00050 through http://www.mayoclinic.com. Accessed February 2009.

(Updated Feb 20, 2009) Avert.org. HIV Testing, p24 antigen test. Available online at http://www.avert.org/hivtesting.htm#q7 through http://www.avert.org. Accessed February 2009.

George E, et. al. Potential of a Simplified p24 Assay for Early Diagnosis of Infant Human Immunodeficiency Virus Type 1 Infection in Haiti. Journal of Clinical Microbiology, October 2007, Pp. 3416-3418, Vol. 45, No. 10.

Sickenger E, et. al. Performance evaluation of the new fully automated human immunodeficiency virus antigen-antibody combination assay designed for blood screening. Transfusion. 2008 Apr;48(4):584-93.

Harrison’s Principles of Internal Medicine. 16th ed. Kasper D, Braunwald E, Fauci A, Hauser S, Longo D, Jameson JL, eds. McGraw-Hill, 2005, P. 1099.

Clarke, W. and Dufour, D. R., Editors (2006). Contemporary Practice in Clinical Chemistry. AACC Press, Washington, DC. Pp. 487-490.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber’s Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby’s Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

Janice K. Pinson MT, MBA. Molecular Business Strategies, Birmingham, MI.