p24 Antigen Test
At a Glance
Why Get Tested?
To check for infection with HIV after a recent exposure or to monitor your body’s response to anti-HIV therapy
When to Get Tested?
If you have been recently exposed to HIV
Sample Required?
A blood sample taken from a vein in your arm or by a finger-prick
The Test Sample
What is being tested?
The p24 test identifies actual HIV particles in blood (p24 is a protein "shell" on the surface of HIV). However, the p24 test is generally only 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. The p24 proteins then become undetectable again after sufficient antibodies to HIV have been produced because they bind to the p24 protein and eliminate it from the blood. Once antibodies are produced, the p24 test will be negative even in people who are infected with HIV. At that point, the standard HIV antibody test will be positive. Later in the course of HIV, p24 protein again becomes detectable.
How is the sample collected for testing?
The collection method depends on the type of test kit used. A blood sample can be collected by finger-prick or by taking blood through a needle placed in a vein in your 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.
The Test
How is it used?
The p24 test may be used to detect early HIV infection and to screen donated blood for HIV. Since p24 levels rise and fall with HIV levels, the test may also be used to monitor anti-HIV therapy and to evaluate disease progression. The advantages of the p24 test are that it can detect HIV infection days earlier, before antibodies develop, and that it is a quantitative test that shows the intensity of HIV expression in the body, which is a measure of how fast the disease is progressing.
When is it requested?
A p24 test may be requested if you have been recently exposed to HIV and wish to know your HIV status, or if you already have HIV and your doctor wants to monitor your response to therapy or to determine if HIV is progressing into AIDS. It is also requested if you are donating blood.
What does the test result mean?
A positive result means that you are infected with HIV: the more p24 protein, the higher the amount of HIV in your blood. A negative test does not exclude HIV infection.
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.
Common Questions
Ask a Laboratory Scientist
Article Sources
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.
