November 25, 2015
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Human Immunodeficiency Virus (HIV) (cont.)

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How do health-care professionals diagnose an HIV infection? What are the different types of HIV tests?

There are two main ways to diagnose HIV infection: detecting the virus directly or detecting antibodies that are made against the virus.

The body makes antibodies to try to fight HIV, although the antibodies cannot eradicate the virus. Antibody testing is often done in two parts. First a sensitive screening test is performed on the blood. If the screening test is positive, a second test is done to confirm that HIV antibodies are present. The types of tests have varied over the years. At first, the screening test used an enzyme-linked immunoassay (ELISA) with confirmatory testing by Western blot. This strategy did not test for HIV-2, sometimes misclassified HIV-2 infection, missed very early infections where antibody had not yet been produced, and sometimes produced indeterminate results. Scientists developed newer tests to address these issues. Other tests took hours or days to return results, requiring people to return to the clinic. Rapid HIV tests (such as Clearview) were developed that could provide results during the initial visit. Some tests can be done at home without the need for a clinic visit. For a self-test or home-test, the person buys a kit (for example, OraQuick), swabs the inside of their cheek, places the swab in the supplied fluid, and reads the results in a test window. Positive results indicate the need to visit a clinic for confirmatory testing.

New fourth-generation tests combine viral detection and antibody detection. Viral detection is done by testing for a component of the virus known as p24 antigen. The fourth-generation tests also detect antibodies against both HIV-1 and the less common HIV-2, as well as antibodies that are made in the early stage of disease (IgM) and in chronic disease (IgG). Draft guidelines from the Centers for Disease Control and Prevention (CDC) in 2012 recommend using FDA-approved fourth-generation tests as the first step in diagnosis. If the fourth-generation test is positive, additional antibody tests are done to differentiate HIV-1 from HIV-2 and to measure viral load.

Viral load is measured by testing the amount of viral RNA in the blood. It can be useful for patients who have confusing test results, such as a positive fourth-generation test but negative or indeterminate individual tests for HIV-1 and HIV-2. In these cases, if viral RNA is detected, the diagnosis of HIV is made. If viral RNA tests are negative, it raises the possibility that the fourth-generation test result was not correct. Viral load is also used to monitor the success of treatment for infected patients. Viral load can also be useful in diagnosing acute retroviral illness because HIV antibodies take time to reach detectable levels.

The CDC recommends HIV testing at least once for all people between the ages of 13 and 64. People at high risk, such as those who use illicit injectable drugs or who have multiple sexual partners, should get tested more frequently. All pregnant women should be tested because effective treatment can dramatically reduce the risk of transmission to the unborn child. Victims of sexual assault should be tested. People who are diagnosed with another sexually transmitted disease, such as syphilis, chlamydia, or gonorrhea, should be tested for HIV as well.

Medically Reviewed by a Doctor on 3/3/2014


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