What is HIV - P24 ANTIGEN?

Also known as: p24 Ag; p24

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 was drawn from a vein in your arm or by a finger-prick

WHAT IS BEING TESTED?

The p24 test identifies actual HIV virus particles in blood (p24 is a capsid structural protein that 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 the sample collected for testing?

The collection method depends on the type of test kit used. A blood sample can be collected by drawing blood through a needle placed in a vein in your arm or by a finger-prick.

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.