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HIV infection is commonly diagnosed by blood tests. There are three main types of tests that are commonly used: (1) HIV antibody tests, (2) RNA tests, and (3) a combination test that detects both antibodies and a piece of the virus called the p24 protein. In addition, a blood test known as a Western blot is used to confirm the diagnosis.
No test is perfect. Tests may be falsely positive or falsely negative. For example, it can take some time for the immune system to produce enough antibodies for the antibody test to turn positive. This time period is commonly referred to as the "testing window period" and may last six weeks to three months following infection. Therefore, if the initial antibody test is negative, a repeat test should be performed three months later. Early testing is crucial because early treatment for HIV helps people avoid or minimize complications. Furthermore, high-risk behaviors can be avoided, thus preventing the spread of the virus to others.
Testing for HIV is usually a two-step process. First, an inexpensive screening test is done. If that test is positive, a second test (Western blot) is done to confirm the result. Antibody tests are the most common initial screening test used. There are different types of antibody screening tests available:
Other tests, such as those that look for virus RNA and the combination test, are not commonly used for screening.
Medically Reviewed by a Doctor on 3/5/2014
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