The most common tests used to diagnose HIV involve looking for HIV antibodies in blood.
The Centers for Disease Control, which provides the national guidelines for HIV testing in the U.S., reports that the average person will develop the antibodies to HIV within 25 days of exposure to the virus. HIV testing looks for the antibodies that you produce to fight off an HIV infection.
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After three months, there's a 97% chance that HIV testing will detect these antibodies, although in rare cases it may take up to six months for antibodies to be found. Therefore, the CDC recommends anyone at risk for HIV infection should be tested six months after possible exposure to HIV. Most people have the test three months after exposure to HIV and then re-test at six months. The CDC website outlines the testing guidelines and can help you decide when to be tested.
The reason for the waiting periods is because the most common tests aren't looking for the HIV virus itself; instead, the tests looks for antibodies produced by the body to fight HIV. Most of the HIV tests use blood to detect HIV infection, but tests using saliva or urine are also available. Some tests take a few days for results, but rapid HIV tests can give results in about 20 minutes. The rapid HIV testing has become the preferred first test used in diagnosing HIV infection. All positive HIV antibody tests must be followed up by another test, the Western blot test, to confirm the positive result. Results of this confirmatory test can take a few days to a few weeks.
When the possibility of acute or early HIV infection is being considered and immediate treatment is needed, tests that can detect both HIV antigens, a protein produced by the virus immediately after infection, and HIV antibodies are used. There is now a rapid test available that identifies both antibodies and antigens within 2 to 4 weeks of being infected. Tests are also available to measure the HIV virus load by looking at HIV RNA.
Getting tested is simple: a blood sample can either be sent to a lab or done in the office with a rapid HIV test on the same day as your visit to the clinic. Home testing kits are also available, but they are really home collecting tests, because the sample is sent to a lab for testing. Testing can be done both confidentially and anonymously, so be sure to ask whether or not your name is associated with your sample if you're concerned about having your test results remain anonymous.
The advantage of being tested by a public health clinic or doctor is the counseling that's provided on test results, prevention, and safe sex practices. Test results can take up to a week, depending on which test is used.