HIV Testing (cont.)

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What is the likelihood of receiving a false positive or false negative with an HIV test?

The current testing protocols are highly accurate but not perfect. The probability of a false result on the test depends on the test and on the likelihood that the person is infected.

Falsely negative tests occur in people who are truly infected with HIV but have negative tests. Among 1,000 people who are truly infected, rapid tests will be falsely negative in zero to six people, depending on the test. Negative antibody tests in people infected with HIV may occur because antibody concentrations are low or because antibodies have not yet developed. On average, antibodies take about four weeks to reach detectable levels after initial infection, and falsely negative tests may occur during this so-called HIV window period. Individuals with negative tests and who had high risk for HIV exposure should be retested in two to three months.

Falsely positive tests occur when uninfected people have positive tests. Among 1,000 people who do not have disease, rapid tests will be falsely positive in zero to nine people, depending on the test. This is the main reason for not relying on a single positive test for diagnosis. As discussed above, all positive initial tests must be confirmed with a Western blot. When both tests are positive, the likelihood of a person being HIV infected is >99%. Sometimes, the Western blot may be indeterminate, meaning that it is neither positive nor negative. In these cases, the tests are usually repeated at a later date or an RNA test is done.

Medically Reviewed by a Doctor on 4/9/2014