Before reading this, we suggest you read "What happens when you get tested for HIV?" That fact sheet has basic information on how HIV testing works, information about the window period, confidential versus anonymous testing, and more. The document you are now reading provides more detailed information on the types of HIV screening tests available.

The ELISA test is designed to be highly sensitive, that is, to miss as few HIV infections as possible. The downside of the high sensitivity is that the ELISA may produce a small number of false-positive tests. This usually is due to the presence of antibodies to other diseases that the ELISA mistakenly recognizes as antibodies to HIV. For this reason, it is important to supplement a positive ELISA with a confirmatory test, such as a Western blot, that is less sensitive but more specific, that is, one that has a lower rate of producing a false-positive result.

Western Blot

The Western blot is the most common test used to confirm positive results from an ELISA or rapid HIV test. It generally is used only as a confirmatory test because it is difficult to perform and requires highly technical skills. Its advantage, however, is that it is less likely to give a false-positive result because it can more effectively distinguish HIV antibodies from other antibodies. However, the Western blot can produce inconclusive results in some samples.

Indirect Fluorescent Antibody (IFA)

The indirect fluorescent antibody (IFA) test also detects antibodies made to fight an HIV infection. As with the Western blot, the IFA test is used to confirm the results of an ELISA. However, it is more expensive than a Western blot test.

You will likely hear this test called a "viral load," "PCR," or "RNA" test. In the public health community, it is also referred to as HIV NAAT (nucleic acid amplification testing). The viral load test usually is used by clinicians to determine whether antiretroviral medications are working to suppress viral replication in HIV-infected persons taking medication. In fact, the viral load test is NOT an FDA-approved method of testing for the presence of HIV infection. Unlike the previously mentioned HIV tests, these types of tests detect the genetic material (RNA) of the virus rather than antibodies to HIV. Therefore, viral load tests can reveal HIV infection during the weeks before antibodies are produced.

When are HIV viral load tests used to detect new infections? They are used to determine whether a baby born to an HIV-infected mother is infected because ELISA tests cannot always make such a determination. Viral load tests also are used when there is a substantial risk for HIV infection from high-risk encounters such as unprotected anal or vaginal sex. HIV viral load tests are not widely available for this purpose, largely because of the cost involved, but also because they can be difficult to interpret. If you are worried that a particular recent sexual episode put you at risk for HIV, you cannot simply go to an HIV testing site and ask for a viral load test. If you are worried, you should go to your doctor, public health clinic, or, in some cases, an HIV testing site and talk to a clinician. He or she can determine your risk for acquiring HIV based on the incident. If the test is available and, in the clinician's opinion, appropriate, a viral load test can be performed. Note that these tests are done on a blood sample drawn from a vein and results can take a few days or even a week or two.

You may wonder why viral load tests are not the standard if they can detect HIV much earlier than antibody tests. First, viral load tests are extremely expensive and require highly trained professionals to interpret the results. Second, the viral load test is so sensitive that a false-positive test is a very real concern. Furthermore, in most cases, antibody tests are sufficient to test for HIV because HIV has been present long enough to be detected. Certainly, we have learned in recent months that a small (but significant) number of people who go for HIV testing do in fact have HIV, but have not yet produced antibodies (which, again, can take up to 3 months to appear), so they assume that they are HIV negative. Researchers are demonstrating ways in which ELISA and HIV viral load tests can be combined in a cost-efficient manner so that such cases are not missed.

You may have heard of "pooled PCR." Researchers in North Carolina have demonstrated a cost-effective and efficient way to combine standard ELISA tests with viral load tests. Instead of performing viral load tests on individual blood samples, they have found a way to combine or "pool" samples and test multiple people's specimens at one time. If a pooled sample tests positive, the laboratory technicians test individual samples in the pool until they find the sample that is positive.

This method allows the benefit of both standard antibody tests and viral load tests to more accurately determine whether someone is HIV infected. If the method catches on across the country, more infections might be identified in the earliest stage, when there is a high likelihood of infecting other people because (a) the viral load is very high, which increases the likelihood of transmitting the virus; and (b) people otherwise may not know they are infected, and may be less likely to practice safer sex. Identifying early infections has the benefit of letting people know sooner and more accurately whether they have HIV infection and has in the indirect benefit of preventing new infections because people who are aware of their HIV status usually take precautions to avoid infecting their partners.

Pooled PCR is not widely available.

In most cases you won't have much choice as to which type of test you receive. Before you get tested, you should understand the difference between an anonymous test and a confidential test. The differences are explained in What happens when you get tested for HIV?" In many places you cannot get an anonymous test, only a confidential test, so if getting an anonymous test is important to you, you will need to find a testing site that can provide one.

Beyond that, the type of ELISA test you receive (eg, blood or oral fluid) varies from test site to test site. If you don't like needles and have a preference for the oral fluid method, check with the test site first to see which type of test is used.

In cities where both rapid antibody tests and standard ELISA tests are available, you do have some choice regarding which one you receive. The rapid test has the advantage of providing immediate results, so you don't have to wait anxiously for several days to learn of the test results. But some people don't have a preference or would rather wait because they need some time to mentally prepare for the test results. If you want a rapid test, check with various testing sites in your city to see where it is available.

Check the National HIV Testing Resources Web site (www.hivtest.org/) for testing sites. The directory provides results based on your ZIP code and shows you what types of HIV tests (anonymous, confidential, and/or rapid) each testing site offers.