HIV Counseling and Testing :: Types of testing

What types of tests are there and how accurate are they?

The body produces antibodies when it becomes infected with HIV. These are markers for the presence of the disease. The main tests used for detecting HIV antibodies are:

• Enzyme-linked immunosorbent assay (ELISA) or Enzyme immunoassay (EIA). This is the first screening test for HIV antibodies. It is performed once for each test; a second ELISA is performed if the first is positive or reactive for antibodies for HIV. This test is an extremely sensitive test and might weed out any antibody that resembles HIV, so there is a possibility that this test alone might result in false positives. That is why there is always a confirmatory test for HIV antibodies that is performed on each repeat ELISA positive result.
• Western blot tests are more specific tests and are used to confirm the EIA/ELISA screening tests. ELISA and Western Blot tests combined can correctly determine HIV presence almost 100% of the time (from blood, oral fluid, or urine.)

For this process, a specimen is sent to the lab to be tested and takes from one to two weeks to complete, depending on where the test is performed.

There are tests that look for the presence of the virus itself rather than antibodies, called PCR or DNA tests. PCR testing is not routinely used. Specialists use these tests when they suspect someone has recently been infected, within the last few weeks, or if the person is experiencing the "acute infection" (fevers, chills, flu like symptoms). This test is also used to test infants born to HIV infected mothers.

The Window Period
Window period is the length of time it takes for a person who has been infected to have enough antibodies in their body to make a positive test result. The average time from infection to a positive antibody test ranges from 6 weeks to 6 months. Some people will test positive within 6 weeks after infection, 90 - 95% of people will test positive 3 months after infection and 99.2% of people will test positive 6 months after infection.

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