|What kinds of HIV screening tests are available in the United States?|
|August 3, 2011|
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.
| HIV Antibody Tests|
Generally referred to as ELISA (enzyme-linked immunosorbent assay) or, alternatively, EIA (enzyme immunoassay), these tests detect HIV antibodies, which the body starts producing between 2 and 12 weeks after becoming infected with HIV. Current HIV antibody tests can detect antibodies as early as 3 weeks after exposure, which is faster than the 1st generation of antibody tests. Current HIV antibody tests are often referred to as 2nd generation (detecting IgG antibodies), 3rd generation (detecting both IgM and IgG antibodies), and 4th generation (detecting both HIV antibody and the p24 antigen, which comes directly from the HIV virus). All positive HIV antibody test results should be confirmed with a Western blot, a rapid test of a different brand than the initial test, or an HIV viral load test. Some HIV antibody tests will not detect HIV-2 (a strain of HIV that is found in western Africa) and some more uncommon strains of HIV-1. If HIV-2 infection is suspected, it is important to know whether the antibody test used is designed to detect both HIV-1 and HIV-2.
Antibody tests may use any of 3 body fluids to detect antibodies to HIV. These are as follows:
- Blood: Drawn from a vein, blood samples are the most common screening method for detecting the presence of HIV antibodies. A test that returns a positive result is confirmed with a follow-up test such as the Western blot before the client is informed of the results (see HIV Antibody Confirmation Tests below).
- Oral Fluid: This test uses oral fluid (not saliva) to detect HIV antibodies in cells found in the mouth along the cheeks and gums. Its reliability is similar to that of the blood test. The fluid is absorbed by a small device (about half the size of a toothbrush), which is held between the cheek and gums for a few minutes and then sent to a lab for processing. There is often much confusion about this test because people mistakenly believe that HIV is transmitted through oral fluids and saliva, which is not the case. As with all ELISA tests, oral tests detect antibodies, not HIV virus itself. OraSure is the only oral fluid test approved by the Food and Drug Administration in the United States. The oral test may have a slightly lower sensitivity than blood-based tests.
- Urine: This test uses a urine sample to detect HIV antibodies (again, not the virus itself) in urine. The accuracy of urine tests is somewhat less than that of blood and oral fluid tests. Positive results must be confirmed with a Western blot.
Results from most ELISA tests and confirmatory Western blot tests are usually available within 2 to 14 days.
| Rapid HIV Test|
Using technology similar to that of an ELISA, a rapid test produces results in approximately 20 minutes. Two types of rapid tests are available now. One uses blood; in this case, a clinician pricks your finger with a small needle and takes a few drops of your blood. The other uses oral fluids and is very similar to the oral fluid test described above.
There are two possible outcomes with this test: negative (meaning that the test does not detect any HIV antibodies) or "preliminary positive." In the case of preliminary positive, the rapid HIV test shows an HIV-positive result, but, as with the ELISA test, that result must be confirmed with a second test such as a Western blot or a second rapid test from a different manufacturer. If the result is preliminary positive, the provider will discuss what this means with the client, including the importance of practicing safer sex and taking other precautions until the confirmation test results come back, and will schedule a time for the client to receive confirmatory results.
| HIV Antibody Confirmation Tests|
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 results. This usually is caused by the presence of antibodies to other diseases that the ELISA mistakenly recognizes as antibodies to HIV. For this reason, it is important to supplement both positive ELISA and rapid antibody results 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 yielding a false-positive result. Sometimes, a rapid antibody test is confirmed with a second rapid test from a different manufacturer.
| 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 yield 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.
| HIV Viral Load Measurements|
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. Unlike the previously mentioned HIV antibody tests, these types of tests detect the genetic material (RNA) of the virus rather than antibodies to HIV.
| Detecting Very Early HIV Infection|
There is a window period with HIV infection, which refers to the time after a person has been infected but before an antibody test result will be positive. Testing for suspected early infections during the window period can be performed using HIV viral load tests or FDA-approved 4th-generation HIV antibody/antigen tests, which detect both HIV antibody and the p24 antigen, which is part of the HIV virus. These tests have the advantage of detecting early HIV infection before antibody development as well as antibodies that are present when chronic infection has been established. HIV viral load testing is also used to confirm infection in babies born to HIV-infected mothers, as antibody testing very early in a baby�s life is not an accurate way to determine whether HIV infection has occurred in the infant.
Identifying early infections has the benefit of letting people know sooner and more accurately whether they have HIV infection, and it has the indirect benefit of preventing new infections because people who are aware of their HIV-positive status usually take precautions to avoid infecting their partners.
If you are worried that a recent sexual or other exposure has put you at risk of HIV infection or if you have symptoms that you suspect could be caused by acute HIV infection, 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 of acquiring HIV based on details of the incident. If the test is available and, in the clinician's opinion, appropriate, a viral load test or 4th-generation HIV test can be performed to assess for very early HIV infection, in addition to an HIV antibody test. 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.
Some laboratories provide pooled viral load testing, a technique allowing blood that is HIV antibody negative to be tested for the presence of HIV virus, thus indicating early infection, without testing each individual sample. However, pooled viral load testing is not widely available.
You may wonder why viral load tests are not the standard if they can detect HIV much earlier than antibody tests. First, in most cases, antibody tests are sufficient to test for HIV because HIV has been present long enough to be detected and current HIV antibody tests can detect HIV infection as soon as 30 days after HIV infection. In addition, viral load tests are not only expensive but also so sensitive that a false-positive result is a very real concern. However, the availability of the 4th-generation HIV antibody/antigen test may make simultaneous testing for both very early HIV infection and chronic HIV infection much more feasible.
| Which Test Is Right for Me?|
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 (for example, 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 results. However, positive results on a rapid test require confirmation. If you want a rapid test, check with various testing sites in your area to see where it is available.
Check the National HIV Testing Resources website (www.hivtest.org/) for the location of 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.