HIV Antibody and HIV Antigen (p24)

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 Terrence Higgins Trust Fastest centres, 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