HIV antibody/antigen (4th generation) test: window periods and results

When HIV enters the body it multiplies quickly, so in the first few weeks the amount of virus in blood can be high. The immune system responds by making antibodies, which Lab Tests Online UK says begin to be detected around 3 to 4 weeks after exposure. The p24 antigen is a protein from the core of the virus that appears earlier. A fourth-generation test measures both, which is why it detects infection sooner than an antibody-only test.

The combination antibody and antigen test is described as the recommended screening test for HIV and is available only as a blood test. It can detect HIV infections in most people by 2 to 6 weeks after exposure, whereas antibody-only tests, which can use blood or oral fluid, detect infection in most people 3 to 12 weeks after exposure.

Testing is advised if you think you may have been exposed, once a year if you are at ongoing risk, if a doctor thinks symptoms may be due to HIV, before or during pregnancy, and for several other situations such as having another sexually transmitted infection. The NHS says some people have a short flu-like illness 2 to 6 weeks after infection, with sore throat, high temperature, swollen glands, muscle aches and rash, but that many people have no symptoms for years and so do not know they have HIV.

Early diagnosis lets treatment begin sooner, slows or stops progression, allows pregnant women to be treated to prevent passing HIV to the baby, and enables people to protect partners. The NHS says that if you are on effective treatment and have an undetectable viral load you cannot pass HIV to someone else.

In the UK the HIV test is free and widely available as a routine test, offered by GPs, sexual health or GUM clinics, antenatal and termination of pregnancy clinics, and drug treatment services. Testing can be anonymous and your GP is not informed of the clinic result without your permission. Blood tests usually return results within three days, and while-you-wait finger-prick tests give a result in minutes.

Point-of-care tests can give false negatives as well as false positives, so positives are always confirmed on blood from a vein. Some tests can also be performed on urine or saliva. Postal self-sampling kits are available through many local sexual health services.

A negative combination test more than six weeks after the last possible exposure is a reliable indication that you do not have HIV. A reactive result needs confirmation: Lab Tests Online UK says further supplementary tests, including tests on a second blood sample, are done because false positives can occur and to determine which type of HIV is present; HIV-1 is the most common type in the UK and worldwide.

Very early in infection a test can be negative before enough antigen or antibody is circulating, so if you had a recent exposure a repeat test is advised after the window period. If you have had a possible exposure within the last 72 hours, the NHS says to go to a sexual health clinic or A&E straight away, where emergency HIV medicine (PEP) may stop infection.

The NHS says HIV is a virus that weakens the immune system; with treatment most people with HIV live a long and healthy life, and in the UK most people being treated do not develop AIDS. HIV is spread through unprotected vaginal, anal or oral sex, sharing injecting equipment, and from mother to baby, not through kissing, hugging, or sharing food or toilet seats. PrEP is available free from sexual health clinics for people at higher risk.

A result of this kind is private medical information, and clinic staff can advise on telling partners without naming you.

If confirmed, you are referred to an HIV specialist team, who check your viral load, CD4 count and other tests and start treatment. If negative, consider ongoing prevention such as condoms and PrEP if you are at risk, and repeat testing as advised.