If the same test is used in a setting with a much higher HIV prevalence of 25% (for example, in the most heavily affected regions of southern Africa), the probability of a negative result being correct is marginally lower, at 99.832%. If this test is used in a setting where 0.2% of people have HIV (for example, among the general population in the UK), the probability of a negative result being correct is 99.998%. See also ‘prevalence’.įor example, take a test with 99.5% sensitivity and 99.5% specificity. Incidence reflects newly acquired infections and conditions. The proportion of people who acquire an infection or develop a condition during a specified period of time. This is expressed by something known to healthcare workers as the ‘negative predictive value’. When very few people among those testing actually have HIV, it is even more likely that a negative HIV test result is accurate. To some extent, the chances of having a false negative result also depend on how common HIV is in your community. There’s more information on sensitivity and specificity on another page. Many modern HIV testing devices are extremely sensitive (over 99%) and extremely specific (over 99%) in relation to long-standing HIV infection. Sensitivity is an indication of test performance when testing people who do have HIV, whereas specificity reflects test performance in relation to people who do not have HIV. The accuracy of a single HIV test depends in part on characteristics known as sensitivity and specificity. False negatives after the window periodĪfter the window period, in people who aren’t taking antiretrovirals, false negative results are extremely rare. For these, tests are less reliable and false negatives may occur.įalse negatives have also been reported in people taking antiretroviral medicines, whether as HIV treatment, pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP). However, in a community with a higher HIV incidence and ongoing transmission of HIV, a higher proportion of the HIV infections which tests need to pick up will be recent infections (that people have only had for a few weeks). For these long-standing infections, tests are extremely accurate and false negatives are therefore extremely rare. In a low incidence setting, with very little ongoing transmission of HIV, most of the HIV infections which tests need to pick up will be infections that people have had for several months or years. The chances of having a false negative result therefore depend on the rate of new HIV infections (incidence) in a community. The latest news and research on types of HIV tests
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