References UK clinical guidance from the NHS, the British HIV Association (BHIVA), and the British Association for Sexual Health and HIV (BASHH). Last updated: April 2026.
The p24 antigen is a protein that forms part of the HIV virus. It appears in the blood within days of infection, long before the body develops detectable antibodies. Modern HIV tests look for both p24 antigen and HIV antibodies, which is why they can shorten the window period compared to older antibody-only tests.
Here’s what p24 is, why it matters, and what it means for your test results.
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What Is p24?
HIV is a virus made up of RNA protected by a core of structural proteins. The most abundant of these is p24 — named because it weighs about 24 kilodaltons. During active HIV replication, large amounts of p24 are released into the blood.
Why It Matters in Testing
Antibody production takes time. The body usually needs 3-8 weeks after infection to produce enough HIV antibodies for standard tests to detect them. p24 antigen appears much earlier — typically within 2-3 weeks of infection. Detecting it allows HIV to be diagnosed sooner.
| Marker | Appears After Infection | Peaks | Fades |
|---|---|---|---|
| HIV RNA (viral load) | 1-2 weeks | 3-4 weeks | Suppressed by ART, never gone |
| p24 antigen | 2-3 weeks | 3-4 weeks | Drops as antibodies take over |
| HIV antibodies | 3-12 weeks | 3+ months | Lifelong |
4th-Generation HIV Tests
4th-generation combined antibody/antigen tests are the UK standard. They detect both p24 antigen and HIV antibodies in a single test. This means:
- HIV can be detected earlier — usually from 4 weeks after exposure
- Very high accuracy once past the window period
- Faster diagnosis means earlier treatment and less onward transmission
Limits of p24 Detection
p24 levels drop once the body starts producing antibodies that bind to and neutralise the antigen. This means a p24-only test (rarely used now) could miss established HIV infections. Combined 4th-generation tests avoid this by looking for both markers at once.
p24 is also not detected by older 3rd-generation antibody-only tests or most rapid home self-tests, which is why the window period for those is longer.
What About Rapid Tests?
Most rapid home HIV tests detect antibodies only. Some newer rapid tests can also detect p24, but they are less sensitive than lab-based 4th-generation tests. For the earliest reliable result, a lab-processed 4th-generation test is the best option.
Read more: Home HIV testing accuracy | HIV test sensitivity and specificity | What is HIV?
Frequently Asked Questions
How soon does p24 antigen appear after infection?
Usually 2-3 weeks after exposure. This is why 4th-generation tests are considered reliable from about 4 weeks post-exposure.
No. p24 detects the viral protein, while viral load tests measure HIV RNA directly. Viral load tests appear even earlier (from around 10-14 days) but are not used for routine screening.
Will p24 still be detectable if I’m on ART?
Effective ART suppresses HIV replication so p24 drops to very low or undetectable levels. This is why 4th-generation tests are used for diagnosis, not for monitoring treatment.
Related reading
- A to Z of Sex: Sexual Health Terms Explained
- Alcohol and HIV: What You Need to Know
- Anxiety and HIV: Coping and Getting Support
- Are All STIs Curable?
- Blood Problems and HIV: Anaemia, Low Platelets and More
Sources
- NHS — Sexual health conditions
- BASHH — UK clinical guidelines for STI management
- British HIV Association (BHIVA) — HIV treatment guidelines
- Terrence Higgins Trust — sexual health information
- UK Health Security Agency — STI surveillance data
Steve Page is a recognised expert on Sexually Transmitted Diseases (STDs) and STD treatments, having published numerous articles in peer-reviewed journals and presented his research at conferences around the world. He has an in-depth understanding of the latest medical research on STDs, and is an advocate for the development of new treatments and protocols to improve the health of those affected. In addition to his research, he has dedicated his career to understanding the causes and symptoms of STDs, as well as how to best treat those impacted.
