The HIV tests that were most commonly used in the past tested for HIV antibodies only. A blood sample is taken through a needle from a vein in the arm. Samples from many individuals are analysed at the same time, in a machine at a laboratory. These tests may also be referred to as “third-generation” tests or as an ELISA (enzyme linked immunosorbent assay). The first- and second-generation tests are no longer in use.

HIV antibodies are not part of HIV itself, but are produced by the human body in response to HIV infection. In the weeks after exposure to HIV, the immune system recognises antigens that belong to HIV and begins to generate HIV antibodies. These antibodies persist for life.

The period during which antibodies are first produced is called “seroconversion”. It is frequently, but not always, accompanied by a set of symptoms commonly called a seroconversion illness, which may be misdiagnosed as flu or glandular fever (or ignored). The most common symptoms are fever, rash, sore throat, swollen lymph nodes, muscle aches and joint pains. When these symptoms appear, they normally do so within six weeks of the HIV exposure.

The “window period” refers to the period after infection with HIV during which tests are not able to detect any HIV antibodies (either because none have been produced yet, or because they are too few in number for the test to pick up). The typical time before a third-generation test can detect infection is thought to be between 20 and 25 days, although it can be longer in some cases.

Except in the case of recent infection, third-generation tests are extremely accurate. For example, a Health Protection Agency evaluation of 16 tests found that all except one had a sensitivity of 100% – in other words, all HIV-positive people tested were correctly diagnosed. Moreover, all had a specificity of 99.8% or over – in other words, if 1000 HIV-negative people were tested, 998 would be correctly diagnosed as such, while two samples would test positive. However in practice, confirmatory tests would be used and individuals would not receive an incorrect positive diagnosis.