Nonetheless, it is estimated that the median window period for fourth-generation tests is 18 days, with half of all infections being detected between 13 and 24 days after exposure. It is hard to say exactly how long the window period for any test lasts, as there are variations between individuals and it is a difficult topic to research (recently infected people would need to know exactly when they were exposed to HIV and then give multiple blood samples over the following days and weeks). The window period is the time immediately after infection, during which tests cannot always detect antibodies or p24 antigen, and so may give a false negative result. There’s information about this test on another page. They include the Bio-Rad BioPlex 2200 HIV Ag/Ab and Elecsys HIV Duo.Īlso available is a rapid, point-of-care combination test, the Alere HIV Combo. They detect the same biomarkers but can also distinguish between samples positive for p24 antigen and for antibodies, and also between HIV-1 and HIV-2. Some tests may also be described as fifth-generation. In product names, ‘Ag/Ab’ indicates that the test looks for both antigen and antibody. Tests of this type include: Abbott Architect HIV Ag/Ab combo, ADVIA Centaur HIV Ag/Ab Combo, Genscreen ULTRA HIV Ag-Ab, VIDAS HIV panel and Vitros HIV combo. The third-generation laboratory test only detects antibodies and is no longer recommended for routine use in the UK. The first- and second-generation laboratory tests are no longer in use. These tests may also be referred to as “fourth-generation” tests or as an ELISA (enzyme linked immunosorbent assay). "Like any screening test, a reactive result (a preliminary positive result) must be verified with confirmatory tests." Samples from many individuals are analysed at the same time, in a machine at a laboratory. A blood sample is taken through a needle from a vein in the arm. The latest news and research on types of HIV testsįourth-generation combination tests are usually performed on plasma or serum (two components of blood, separated from whole blood using laboratory equipment). Since p24 antigen is usually detectable a few days before HIV antibodies, a test that can detect p24 has a slightly shorter window period than a test that only detects antibodies. High levels of p24 are present in the blood during the short period between HIV infection and seroconversion, before fading away. These antibodies persist for life.Īn HIV antigen, known as p24, is a structural protein that makes up most of the HIV viral core. In the weeks after exposure to HIV, the immune system recognises some components of the virus and begins to generate HIV antibodies in order to damage, neutralise or kill it (this period is known as ‘seroconversion’). HIV antibodies are produced by the human body in response to HIV infection. They are “fourth-generation” tests, with more advanced technology than the earlier generations. They are called combination or combo tests because they detect both substances. The most accurate and reliable routinely used diagnostic HIV test is a laboratory test that can detect both HIV antibodies and p24 antigen.
Their sensitivities and specificities are excellent.They test for p24 antigen as well as antibodies, so are better suited for detecting recent infection.These are the recommended tests for routine use in the UK.
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