Fourth-generation tests

  • These are the recommended tests for routine use in the UK.
  • They test for p24 antigen as well as antibodies, so are better suited for detecting recent infection.
  • Their sensitivities and specificities are excellent.
  • Like any screening test, a reactive (positive) result will be confirmed with one or two supplementary tests.

Fourth-generation tests, introduced in the late 1990s, are ELISAs which test for both HIV antibodies and p24 antigen. It is essentially a third-generation ELISA, to which is added an ELISA for p24 antigen.

They may also be called combined antigen/antibody tests. In product names, 'Ag/Ab' indicates that the test looks for both antigen and antibody.

Testing for p24 detects some cases of HIV infection before antibodies are produced, shortening the window period in which people may test negative despite having infection.

Not all tests distinguish between samples that are positive for antibody and samples that are positive for p24. Therefore, although they may help identify people with primary infection, they do not distinguish between primary and chronic infection.

Fourth-generation tests are recommended for use as a first-line assay in the UK guidelines.1,2,3 Most commercially available tests are able to detect HIV-2 and diverse HIV-1 strains such as subtype O.

Blood samples are normally used, although some kits can also be used with saliva.

These tests are less commonly used in the United States. So far, only Abbott’s Architect Ag/Ab Combo test is approved by the FDA.

Like any screening test, a reactive (positive) result will be confirmed with one or two supplementary tests.

Related Links


  1. British HIV Association, British Association for Sexual Health and HIV, Children’s HIV Association. Don’t forget the children: Guidance for the HIV testing of children with HIV-positive parents. Available at (accessed 29 April 2010), 2009
  2. MedFASH and BASSH Standards for the management of sexually transmitted infections (STIs)., 2010
  3. Health Protection Agency Anti-HIV Screening – minimum testing algorithm. National Standards Methods VSOP 11. Retrieved January 19, 2012, from, 2009
This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.
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