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p24 Antigen Blood Test

Last updated Jan. 14, 2019

Approved by: Krish Tangella MD, MBA, FCAP

The p24 Antigen Blood Test is a test to assess the levels of p24 in blood. It indirectly measures the levels of the HIV virus, which is also called the viral load.


What are the other Names for this Test? (Equivalent Terms)

  • HIV Antigen (p24) Blood Test
  • p24 Core Protein Antigen Blood Test
  • p24 Protein Blood Test

What is p24 Antigen Blood Test? (Background Information)

  • p24 is a protein component of human immunodeficiency virus (HIV). Specifically, it is present in the virus’ outer shell, or capsid. p24 is used as a marker for HIV infection; the levels of p24 directly correlate with the levels of HIV
  • HIV is spread through blood and sexual fluids. Upon entering the host, HIV rapidly proliferates, which causes a spike in p24 levels. Flu-like symptoms emerge as the immune system reacts to the foreign invader
  • After some time, the immune system manages to contain the HIV virus. Thus, the levels fall; this is mirrored by falling levels of p24. However, the virus survives through what is known as the latent phase of infection
  • The HIV hibernates inside a specific type of white blood cell called a helper T cell. After a period of time, HIV re-emerges. When this happens, the HIV bursts open the T cell, releasing more of the virus. In this way, levels of helper T cells decline dramatically
  • The immune system begins producing antibodies against HIV soon after it encounters the virus. Antibodies are bodily defense proteins meant to clear such foreign invaders
  • The recognition targets of antibodies are called antigens; p24 is one such target in the case of HIV. Antibodies to p24 can be isolated and their levels gauged. This is the principle of the p24 Antigen Test
  • The p24 Antigen Blood Test is a test to assess the levels of p24 in blood. It indirectly measures the levels of the HIV virus, which is also called the viral load

What are the Clinical Indications for performing the p24 Antigen Blood Test?

Following are the clinical indications for performing a p24 Antigen Blood Test:

  • Routine screening for HIV, both in susceptible individuals and in those aged 13-64 years who are at an increased risk
  • As a follow-up to an HIV antibody test, if done separately
  • Monitoring progression of a known HIV infection
  • Monitoring anti-retroviral therapy
  • Frequent and persistent infections
  • Infection by pathogens that are normally trivial, including pathogens such as yeasts that cause Candida albicans (oral/genital candidiasis, thrush)

How is the Specimen Collected for p24 Antigen Blood Test?

Following is the specimen collection process for p24 Antigen Blood Test:

Sample required: Blood

Process of obtaining blood sample in adults:

  • A band is wrapped around the arm, 3-4 inches above the collection site (superficial vein that lies within the elbow pit)
  • The site is cleaned with 70% alcohol in an outward spiral, away from the zone of needle insertion
  • The needle cap is removed and is held in line with the vein, pulling the skin tight
  • With a small and quick thrust, the vein is penetrated using the needle
  • The required amount of blood sample is collected by pulling the plunger of the syringe out slowly
  • The wrap band is removed, gauze is placed on the collection site, and the needle is removed
  • The blood is immediately transferred into the blood container, which has the appropriate preservative/clot activator/anti-coagulant
  • The syringe and the needle are disposed into the appropriate “sharp container” for safe and hygienic disposal

Preparation required: No special preparation is needed prior to the test. 

What is the Significance of the p24 Antigen Blood Test Result?

  • A positive test result may indicate infection with HIV
  • The p24 Antigen Blood Test is rarely performed as a stand-alone test. Instead, it is used alongside the HIV antibody blood test

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results based on the overall clinical scenario. 

Additional and Relevant Useful Information:

  • Another useful marker for HIV infection is gp41 antigen

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider of the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

References and Information Sources used for the Article:


Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: Dec. 2, 2014
Last updated: Jan. 14, 2019