Philip Cunningham

NSW State Reference Laboratory for HIV, St Vincent’s Hospital, Sydney NSW

Last reviewed: May 2019

The diagnosis of human immunodeficiency virus-1 (HIV-1) and human immunodeficiency virus-2 (HIV-2) infection is usually made by the detection of circulating antibodies in blood. Antibodies are identified by the use of a screening test (standard test), usually an enzyme immunoassay (EIA), followed by definitive diagnosis using test strategies that include supplemental EIAs, nucleic acid tests and a Western Blot assay (reference test). Reference testing strategies and algorithms are designed to provide the greatest positive predictive value through a combination of tests. In some situations, such as pre-seroconversion (acute infection) or neonatal infection, detection of HIV antibodies may be unreliable. In these cases, diagnosis of infection may require tests that detect HIV directly – whether by quantitating plasma HIV RNA or HIV proteins (p24 antigen), or by detection of HIV DNA in blood mononuclear leucocytes. In 2005, screening tests became available that detect both HIV antibody and p24 antigen in a single test, which improved the detection of individuals presenting with acute HIV infection. Combined antibody and antigen tests are so called fourth generation tests and are widely used as screening (standard) tests to detect antibodies to both HIV-1 and HIV-2. HIV-2 is endemic in West Africa and in some countries with historical socioeconomic links with West Africa (e.g. France, Spain, Portugal, and former Portuguese colonies such as Brazil, Angola, Mozambique, and parts of India). HIV-2 is not prevalent in the Australasian Pacific region but has been reported particularly in individuals with links to HIV-2 endemic countries. HIV-2 is similar to HIV-1 in its structure, genomic organisation, route of transmission and ability to cause acquired immune deficiency syndrome (AIDS). Because the viruses are closely related there is significant serological cross-reactivity in antibody tests so the use of specific HIV-2 Western Blot assays and nucleic acid tests are used to confirm the diagnosis.