p24 Antigen

To check for infection with HIV after a recent exposure

If you have been recently exposed to HIV

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

None

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.

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

No test preparation is needed.

This form enables you to ask specific questions about your tests. Your questions will be answered by a laboratory scientist as part of a voluntary service provided by one of our partners, American Society for Clinical Laboratory Science. If your questions are not related to your lab tests, please submit them via our Contact Us form. Thank you.

Conditions: HIV
In the News: Pediatricians Endorse HIV Screening for All Teens Who Live in High Risk Areas (2011), FDA Approves First Combined HIV Antibody and Antigen Test (2010)

Centers for Disease Control and Prevention: HIV testing
AVERT.org: HIV testing, Frequently Asked Questions

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.