How accurate are HIV tests?

Standard HIV antibody (ELISA) tests are at least 99.5% accurate when it comes to detecting the presence of HIV antibodies. This high level of sensitivity however means that their specificity (ability to distinguish HIV antibodies from other antibodies) is slightly lowered. Once an individual is out of the window period, it is more likely that they will receive a false positive result than a false negative.

Any HIV positive result given by an ELISA test must therefore be confirmed using a second test. Secondary tests include:

• Western Blot Assays ? One of the oldest but most accurate confirmatory antibody tests. It is complex to administer and may produce indeterminate results if a person has a transitory infection.
• Indirect Immunofluorescence Assay ? Like the Western blot, but uses a microscope to detect HIV antibodies.
• Line Immunoassay - Commonly used in Europe. Reduces chance of sample contamination and is as accurate as the Western Blot.
• A second ELISA ? In resource-poor settings with relatively high prevalence, a second ELISA test may be used to confirm a diagnosis. The second test will usually be a different commercial brand and will use a different method of detection to the first.

When two tests are combined, the chance of getting an inaccurate result is much less than 0.1%.