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HIV/AIDS (cont.)

What Tests Do Health-Care Professionals Use to Diagnose HIV/AIDS?

HIV infection is commonly diagnosed by blood tests. Testing for HIV is usually a two-step process. First, a screening test is done. If that test is positive, a second test (Western blot) is done to confirm the result.

There are three common types of screening tests that use a blood specimen:

  1. HIV antibody tests;
  2. a fourth-generation combination antibody/antigen test that detects both antibodies and a piece of the virus called the p24 antigen;
  3. RNA tests (HIV RT PCR or viral load);
  4. in addition, a blood test called a Western blot is necessary 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 "window period" and may last six weeks to three months following infection. The antigen/antibody assay is most sensitive and may be positive within two weeks after infection. If the initial antibody test is negative or unclear, a repeat test should be performed three months later.

Other tests can detect antibodies in body fluids other than blood, such as saliva, urine, and vaginal secretions. Some of these are designed to be rapid HIV tests that produce results in approximately 20 minutes. These tests have accuracy rates similar to traditional blood tests. OraQuick is an at-home test that uses an oral swab to detect HIV antibodies in oral fluid. Clearview is another rapid HIV test that can detect HIV antibodies in blood or plasma. HIV home-testing kits are available at many local drugstores. Blood is obtained by a finger prick and blotted on a filter strip. Other test kits use saliva or urine. The filter strip is mailed in a protective envelope to a laboratory to be tested. Results are returned by mail within one to two weeks.

All positive HIV screening tests must be confirmed with a confirmatory blood test called the Western blot to make a positive diagnosis. If the screening test and the Western blot are both positive, the likelihood of a person being HIV infected is >99%. Sometimes, the Western blot is "indeterminate," meaning that it is neither positive nor negative. In these cases, the tests are usually repeated at a later date. In addition, an RNA test for the virus might be done. Because the p24 antigen is present in the blood before the body forms antibodies, the antibody/antigen screening test may decrease the "window period" and allow for earlier detection of HIV infections.

RNA testing (viral load test) detects HIV RNA in the blood. It is not commonly used for screening but can be helpful in detecting early HIV infection when a person is in the window period or if the screening tests are unclear.

Medically Reviewed by a Doctor on 8/24/2016

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Read What Your Physician is Reading on Medscape

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