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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

At a Glance

Why Get Tested?

Most often as part of a combination HIV antibody/antigen test, to screen for infection with Human immunodeficiency virus (HIV); less commonly, as an individual test to screen for HIV after a recent exposure

When to Get Tested?

As part of a combination test, at least once if you are age 13-64 years; annually if you are at increased risk for HIV; as an individual test, soon after you think you have been exposed to HIV

Sample Required?

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

Test Preparation Needed?


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.

The Test

Common Questions

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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

Galbraith, J. and Edwards, A. (Updated 2011 August 5). Fourth-Generation HIV Tests. Medscape Reference [On-line information]. Available online at through Accessed February 2012

Rollins, G. (2010 November). What's the Best Testing Strategy for HIV Infection? AACC Clinical Laboratory News v36 (11). [On-line information]. Available online at through Accessed February 2012.

Bennett, N. (Updated 2011 July 14). Laboratory Assays in HIV Infection. Medscape Reference [On-line information]. Available online at through Accessed February 2012.

Rivera, D. et. al. (Updated 2012 February 1). Pediatric HIV Infection. Medscape Reference [On-line information]. Available online at through Accessed February 2012.

Pagana, K. D. & Pagana, T. J. (© 2011). Mosby's Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 609-612.

Douglas, D. (2012 February 1). 'Determine' HIV Combo Test Fails at Spotting Acute HIV. Medscape News Today from Reuters Health Information [On-line information]. Available online at through Accessed February 2012.

Delaney, K.P. et. al. Performance of an alternative laboratory-based algorithm for HIV diagnosis in a high-risk population. J Clin Virol (2011), doi:10.1016/j.jcv.2011.09.013.

Masciotra, S. et. al. Evaluation of an alternative HIV diagnostic algorithm using specimens from seroconversion panels and persons with established HIV infections. J Clin Virol (2011), doi:10.1016/j.jcv.2011.09.011.

Wesolowski, L. et. al. Performance of an alternative laboratory-based algorithm for diagnosis of HIV infection utilizing a third generation immunoassay, a rapid HIV-1/HIV-2 differentiation test and a DNA or RNA-based nucleic acid amplification test in persons with established HIV-1 infection and blood donors. J Clin Virol (2011), doi:1016/j.jcv.2011.09.026.

Clarke, W., Editor (© 2011). Contemporary Practice in Clinical Chemistry 2nd Edition: AACC Press, Washington, DC. Pp 609-612.

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.

(Dec. 1, 2007) Mayo HIV Testing, What tests and when to get tested. Available online at through Accessed February 2009.

(Updated Feb 20, 2009) HIV Testing, p24 antigen test. Available online at through 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.