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What is being tested?

The p24 test identifies actual HIV virus particles in blood (p24 is a capsid structural protein which makes up a protein 'shell' on the surface of the HIV virus). However, the p24 test is generally only positive from about two to three 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 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.

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 result mean?

A positive result means that you are likely to be infected with HIV, and further antibody testing is required to confirm this diagnosis. A negative test does not exclude HIV infection.

Is there anything else I should know?

The stand-alone 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.

Testing for the p24 antigen is incorporated into most modern HIV antibody tests, which are routinely used to test for HIV infection in most Australian laboratories. The incorporation of p24 antigen means that these tests are able to detect the presence of HIV infection earlier after the time of infection than before. When these tests are positive the presence of HIV is always confirmed by testing by another method.

All tests for HIV infection require informed consent and pre-test counselling, which is a discussion between doctor and patient about the test and its implications.

Common questions

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

Yes, in resource limited settings, rapid tests from a finger prick blood sample or saliva may be performed. However in Australia, the HIV antibody test is the preferred test for HIV.

  • What are the symptoms of HIV infection?

If you believe you have been recently infected with HIV, the first symptoms may be similar to a flu-like illness and include, fever, fatigue, mouth ulcers, skin rash, sore throat and swollen lymph glands. However, many people who have HIV do not experience symptoms for many years, and the symptoms they experience can be very similar to symptoms of other illnesses. The only reliable way to tell if you are infected with HIV is to get tested.

More information

RCPA Manual: HIV p24 antigen

Last Updated: Thursday, 1st June 2023

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