HIV Infection Worries: What's the Best Test?

Q: I'm worried I may have been exposed to HIV. Are there any tests that can tell me immediately if I've been infected? How accurate are they? What's the most accurate test currently available?

A: Yes, it is possible to get a rapid test for HIV—with a result in as little as 20 minutes. Rapid HIV testing can be done with a blood test or using saliva collected by swabbing the inside of the mouth. The blood test is slightly more accurate.

However, before rushing out to get tested, you need to be aware of some limitations of the tests. The screening tests use a technique called an enzyme immunoassay (EIA). EIA tests detect the presence of specific antibodies you body makes when exposed to HIV.

It may take as long as six weeks before there are enough antibodies in your blood or saliva to give a positive EIA test. So if you were recently infected you may test negative, but still have the virus. Also you need to be aware that a rapid test may be positive even when a person does not have HIV infection. This is called a false positive. It occurs in about 3 out of 1,000 tests.

False positives also can occur with more traditional blood tests done in a laboratory. The difference is that the "traditional" test result is not reported until a second, more accurate confirmatory test is done.

If any initial test using antibodies, either traditional or rapid, is positive, then a Western blot test is done to confirm the diagnosis. Results of this test may not be available for several days. The Western blot test is the most specific antibody test for HIV. There are several causes of false positive EIAs; but a false positive Western blot is extremely rare.

Neither the EIA nor the Western blot is accurate immediately after a person is infected with the HIV virus. It can take six weeks or more for these tests to become positive. The period between infection with HIV and the development of positive test for antibodies is called "the window period." This term refers to the window of time between getting the HIV infection and the ability to detect the body's response to infection (the development of antibodies).

The other test used to determine HIV infection is called a viral load test. The test detects the presence of actual virus in the blood rather than testing for an antibody. It is not recommended for general screening. It can be ordered if a person has symptoms suggestive of a newly acquired HIV infection (such as fever, sore throat, headache, body aches accompanied by possible exposure to the virus).

Also on This Topic:

• Life Expectancy after HIV Infection: Getting Better All the Time
• Fighting the Virus: Treatment Options for HIV and AIDS
• HIV and AIDS: From Flu-Like Symptoms to Life-Threatening Infections
• How HIV Spreads: Myths and Facts