HIV Antibody and HIV Antigen (p24)
Note: this site is for informational purposes only. To view test results or book a test, use the NHS app in England or contact your GP.
The HIV antibody and HIV antigen (p24) test measures both HIV antibodies and the p24 antigen in the blood using a blood sample taken from a vein in the arm or sometimes a finger-prick sample. It is used to screen for and diagnose HIV infection, including early infection, as the p24 antigen can be detected before antibodies are fully formed.
Why get tested?
To determine if you are infected with human immunodeficiency virus (HIV)
When to get tested?
When you think you may have been exposed to the virus; once a year if you are at risk of being exposed to the virus; when you doctor thinks your symptoms may be due to HIV; before becoming pregnant or during pregnancy.
Sample required?
A blood sample collected from a vein in your arm or from a fingerprick; some tests can also be performed on urine or saliva (spit).
Test preparation needed?
None
What is being tested?
Human immunodeficiency virus (HIV) infects the cells of a person’s immune system and is the cause of AIDS (acquired immunodeficiency syndrome).
When a person becomes infected with HIV, through exposure to the blood or body fluids of an infected individual, the virus begins to reproduce very rapidly. So, during the first few weeks of infection, the amount of virus (viral load) in the blood can be quite high.
The immune system responds by producing antibodies directed against the virus and these begin to be detected in the blood around 3–4 weeks after exposure to the virus. As the level of HIV antibody increases, the viral load in the blood decreases.
This early HIV infection may cause no symptoms or sometimes a flu-like or glandular fever-type illness. The only way to determine whether a person has been infected is through HIV testing. Modern HIV screening tests detect HIV antigens (parts of the virus itself, usually a protein called the p24 antigen) and/or antibodies produced in response to an HIV infection.
Two main test types are available for HIV screening:
- Combination HIV antibody and HIV antigen test— this is the recommended screening test for HIV and is available only as a blood test. By detecting both antibody and antigen, the combination test increases the likelihood that an infection is detected soon after exposure. These tests can detect HIV infections in most people by 2–6 weeks after exposure.
- HIV antibody testing— This test takes a little longer to become positive after an exposure but can be carried out on blood or oral fluid. HIV antibody tests can detect infections in most people 3–12 weeks after exposure.
How is the sample collected for testing?
Blood is taken through a needle placed in a vein in your arm or sometimes by finger prick. Saliva is collected using a spatula with an absorbent pad on its tip which is swept around between the cheeks and the upper and lower gums.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
Common questions
HIV antibody and antigen testing is used to screen for and diagnose HIV infections. Over time, if not diagnosed and treated, HIV destroys the immune system and leaves a person’s body vulnerable to debilitating and life-threatening infections – the symptoms of AIDS (you can read more about this in the article on HIV Infection and AIDS.).
Detecting and diagnosing HIV early in the course of infection is important because:
- It allows for early treatment that slows or stops progression to AIDS.
- An individual can modify behaviour so as to prevent the spread of disease.
- A pregnant woman can undergo treatment that prevents HIV infection from passing to her child.
Regardless of the type of screening test used, a positive result requires follow up with supplemental testing to confirm a diagnosis of HIV, because false positives can occur. Further testing is also used to determine the type of HIV virus present. There are two types of HIV. HIV‑1 is the most common type found in the UK and worldwide, while HIV‑2 occurs mainly in parts of Africa.
Testing for HIV should be requested if you think you may have been exposed to HIV (for example if a sexual partner is HIV positive). Testing is also recommended if:
- You are sexually active (three or more sexual partners in the last 12 months).
- You received a blood transfusion before 1985 (since 1985 all blood donated in the UK and the US is tested for HIV and infection by transfusion is now exceptionally rare).
- You are uncertain about your partner’s sexual behaviour with others.
- You are a male who has had sex with another male.
- You have used street drugs by injection, especially when sharing needles or other equipment.
- You have a sexually transmitted disease (STD) or an infection with hepatitis B or C.
- You are a health care worker with direct exposure to blood at work.
- You are pregnant.
- You are a woman who wants to make sure you are not infected with HIV before becoming pregnant.
- You must have a test if you want to become a blood donor, use your sperm for artificial insemination, have an egg implantation or adopt a child.
In the UK the HIV test is free, and widely available as a routine test. You can be tested by your doctor, at any clinic dealing with STDs such as Genito-Urinary Medicine (GUM) clinics, at antenatal or termination of pregnancy clinics and at drug treatment or needle-exchange centres.
Test results on blood samples should not take longer than three days. While-you-wait (so-called “point-of-care”) testing using a finger prick test can give a result in a few minutes. However, these tests can give false negative as well as false positive results. Positives should always be confirmed with tests on blood taken from a vein.
Testing can be carried out anonymously and your GP is not informed of the clinic result without your permission. Clinic staff will give advice about informing sexual partners if your test is positive.
An uninfected person’s blood will not contain the HIV virus or antibodies to HIV. An HIV antigen/antibody test will be negative.
Sometimes, an uninfected person’s blood will cause some non-specific reaction in the screening test. Further supplementary HIV tests, including tests on a second blood sample, will normally be carried out to confirm that there is no true HIV infection. A person with HIV infection will have a reactive result on the HIV screening test. Further supplementary HIV tests, including tests on a second blood sample, will also be reactive, confirming a positive result.
Rarely, a person who is just developing a very early HIV infection might not yet have the HIV virus or antibodies to it circulating freely in the blood. An HIV antigen/antibody test might be negative. People who have been exposed to HIV very recently and who have a negative HIV screening result are advised to be re-tested 8–12 weeks after exposure.
HIV antigen/antibody testing will not detect HIV immediately after exposure, during the window before the development of a high level of virus and/or antibodies in the blood. Your result may be negative despite the fact that you are infected – a false negative interpretation. Because of this, repeat testing is important. You should have another HIV test at least 2–3 months from the time of a possible exposure to the virus.
The only reliable way to tell if you are infected with HIV is to be tested. This is because there may be no symptoms of the initial infection, or they may be very similar to symptoms of other viral illnesses such as flu, with fever, sore throat, tiredness and muscle aching. Sometimes there are swollen glands and a rash. Many people with HIV do not experience symptoms for years after the initial infection.
The typical symptoms of the late stage of HIV infection, AIDS, are described by NHS. Many people with HIV do not experience symptoms of immune deficiency for many years after infection.
Currently, there is no cure for HIV or AIDS. However, there are treatments that can greatly improve health and restore a normal life expectancy. Information about emergency HIV drugs and long term antiretroviral drugs is available from NHS. Early diagnosis of HIV infection is important to prevent its transmission to others. There is currently no vaccine to protect against HIV, but avoiding high-risk activities such as having unprotected sex and sharing needles for injecting drugs can help to prevent its spread.
Drug treatment can sometime be given to a person very quickly after they are exposed, in order to prevent the virus from establishing an infection (this is known as “post-exposure prophylaxis”).
Yes. If your test is positive for HIV, it is wise to tell your healthcare providers to make sure you receive the right treatment for any other health issues. It is important that you tell all current and future sex partners and anyone with whom you share needles. Counselling services can help you to inform the people who need to know.
No. Because maternal antibodies are transferred from mother to baby and stay in the newborn’s system for 6–12 months, a different test must be used. This test is called the HIV proviral DNA test.