p24 Antigen Test

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Also known as: p24 test
Formal name: Protein 24 capsid 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

Test Preparation Needed?

None

The Test Sample

What is being tested?

The p24 antigen test identifies actual HIV particles in blood (p24 is a protein "shell" on the surface of HIV). However, the p24 test is generally positive only from about one week to 3 or 4 weeks after infection with HIV. The p24 protein cannot be detected until about a week after infection because it generally takes that long for the virus to multiply sufficiently to be detected. The p24 protein then becomes undetectable again after sufficient antibodies to HIV have been produced to bind the p24 protein and eliminate it from the blood. At that point the standard HIV antibody test becomes positive. Often years later in the course of HIV, when AIDS (acquired immunodeficiency syndrome) develops and antibody production falls, the p24 protein may again become 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 be taken 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.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

Ask a Laboratory Scientist

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.