Tip - Understanding test results - False negatives and false positives

The ideal HIV screening assay would correctly identify all HIV-positive and HIV-negative individuals 100% of the time. While real-world procedures are extremely accurate, they do not achieve 100% accuracy. Inaccurate test results fall into two categories: falsely negative and falsely positive. A false negative result is one that fails to detect antibodies or antigen in a person who is in fact HIV-infected (i.e., a result which incorrectly identifies an HIV-positive individual as HIV-negative). This is most likely to occur during the window period, when antibodies and antigen are not yet present at detectable levels. Conversely, a test that incorrectly returns a positive result in an HIV-negative person is known as a false positive. This may happen if non-HIV antibodies are incorrectly identified as antibodies to HIV. Because of the risk that a positive result from a single test is, in fact, a false positive, many doctors prefer to talk about the result being 'reactive' rather than 'positive'. The result will need to be replicated in confirmatory testing before an HIV-positive diagnosis can be made. Rates of false positive and false negative results are related to test characteristics known as sensitivity and specificity.