False negatives and false positives

  • A false negative result is a failure to correctly identify an HIV-infected person as HIV-positive.
  • A false positive result is an incorrect HIV-positive diagnosis in a person who is not actually infected.
  • True positive and true negative results are correct identifications of the presence and absence of HIV, respectively.

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.

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 It may have been superseded by more recent developments.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.