Detection of HIV (human immunodeficiency virus or the AIDS virus) by fourth generation technologies combines the detection of a virus protein (p24 antigen) with the search for antibodies produced by the body in response to the presence of the two main sub-types of the virus (HIV 1 and HIV 2).
A negative result for the anti-HIV 1/2 and p24 combination does not definitively rule out the presence of HIV infection. However, a negative result indicates that there was no evidence of the disease at the time of the test. Repeating the test (at least annually) is still recommended for certain groups of higher-risk individuals.
Most infected individuals should show positive results within 2 to 3 weeks. Over 95% of specimens are expected to be positive within 6 weeks and 99% after 12 weeks. There is still a slight chance of false negative results if the samples were taken too soon after infection. All positive results obtained with the anti-HIV 1/2 and p24 screening technique must be confirmed by a more complex technique (western blotting) performed only by the Laboratoire de Santé publique du Québec (LSPQ) before a confirmed “positive” result is issued.