False negative and false positive tests

Generally speaking, a false negative result is a failure to correctly identify an HIV-infected person as HIV-positive and 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.

Although the rate of false positive and false negative HIV test results is low, they do sometimes occur. Their incidence is largely influenced by a number of factors, including the type of test used, the limitations of current testing technologies and even the time at which a person is given a test.

Most HIV tests work by detecting HIV antibodies in either a blood or saliva sample. Therefore, in a false-negative result, the test either fails or is unable to detect HIV antibodies. This most often occurs not because of the test itself, but because the HIV-infected person is tested prematurely during the window period. It is during this period that the immune system has not produced enough HIV antibodies to register a detectable response by rapid test kits.

By contrast, a false positive occurs when an HIV test incorrectly identifies a non-HIV-antibody as being an HIV antibody. On the rare occasion when this does occur, it is usually because antigens similar to those of HIV are detected. Some diseases, like lupus, have been known to elicit such a response.

Within a general population, the rate of false negatives and false positives is largely determined by the sensitivity and specificity of the test used.

Sensitivity is defined as the percentage of true positives who are actually identified as such. Lower rates of sensitivity equate to more false negative results.

Specificity is defined as the percentage of true negatives who are actually identified as such. Lower rates of specificity equate to more false positive results.

In resource-limited settings, HIV diagnosis is done with rapid diagnosis tests (RDT) using two or three different RDTs (according to national guidelines). Rapid tests allow scale up and decentralisation of treatment, both of which are essential to saving lives. They work well for screening blood transfusions and identifying people who need further tests.

Current generation HIV tests are considered extremely accurate. The blood-based HIV ELISA has a demonstrated sensitivity of between 99.3 percent to 99.7 percent, with a specificity of between 99.91 percent and 99.97 percent. Newer, combination, which test for both HIV antibodies and antigens, are reported to have a clinical sensitivity of 99.9 percent.