What are the different types of HIV testing?
There are three main types of HIV tests: antibody tests, RNA (viral load) tests, and a combination test that detects both antibodies and viral protein called p24 (antibody-antigen test, or HIV Ab-Ag test). All tests are designed to detect HIV-1, which is the type of HIV in the United States. Some antibody tests and the combination test can also detect HIV-2 infections, which are usually limited to West Africa. No test is perfect; tests may be falsely positive or falsely negative or impossible to interpret (indeterminate, see below).
Positive test results are reportable to the health department in all 50 states and include the patient's name. This information is then reported to the CDC (without names) so that the epidemiology and infection spread rates can be monitored. The names sent to the state remain confidential and will not be reported to employers, family members, or other such people. Some states allow anonymous testing in which the patient's name is not recorded.
HIV antibody tests: HIV possesses many unique proteins on its surface and inside the virus itself. When someone is infected with HIV, their body produces proteins designed to tag the virus for elimination by the immune system. These proteins are called antibodies, and they are directed against the unique proteins of HIV. Unfortunately, these HIV antibodies do not eliminate the virus, but their presence serves as a marker to show that someone is infected with HIV. HIV antibody tests are the most commonly used tests to determine if someone has HIV.
Antibody testing is usually done on a blood sample, often using an enzyme-linked assay called an ELISA or EIA. In this test, a person's serum is allowed to react with virus proteins that have been produced in the laboratory. If the person has been infected with HIV, the antibodies in the serum will bind to the HIV proteins, and the extent of this binding can be measured. Negative EIA results are usually available in a day or so.
There are some rapid HIV testing kits on the market that can be used in a health care professional's office or other points of care. Most of these kits still require blood to be drawn, although it can be done using a simple finger stick in some cases. Home-testing is also possible and may be more convenient for some individuals. Home testing is done by adding a drop of blood to a test strip and mailing the sample to a laboratory. The FDA has also approved kits that test for antibodies in saliva (oral fluid) instead of blood. Oral fluid is obtained by swabbing the gums. Some of the newest tests are done on urine, although results may be less accurate than results from blood.
Because there is a small chance that a person's antibodies will falsely attach to the non-HIV proteins during the test, a second, more specific test is done on all initially positive tests. This second test is called the Western blot test. In this test, the HIV proteins are separated by size and electric charge and the person's serum is layered on the test strip. If the test is positive, a series of bands are detected that indicate specific binding of the person's antibody to specific HIV virus proteins. This test is only done to follow up an initially positive screening test. It is not as helpful when performed on its own.
HIV RNA tests: The HIV RNA is different than all human RNA, and tests have been developed to detect HIV RNA in a person's blood. Because this test can be used to estimate the amount of circulating HIV in the blood, it is often referred to as an HIV viral load. This uses a type of test called a polymerase chain reaction (PCR). These tests are important for newborn screening of HIV-positive mothers since maternal antibody may cross the placenta and be present in the newborn. These tests may also be helpful in detecting HIV infection in the first four weeks following exposure, before antibodies have had time to develop. However, they are costly and are not routinely used to screen for infection.
HIV antibody-antigen (Ab-Ag) test: The HIV Ab-Ag test detects antibodies directed against HIV-1 or HIV-2, as well as a protein called p24, which forms part of the core of the virus (an antigen of the virus). This is important because it takes weeks for antibodies to form after the initial infection, even though the virus (and the p24 protein) is present in the blood. Thus, Ab-Ag testing may allow for earlier detection of HIV infections. Preliminary studies suggest that diagnosis could be made an average of one week earlier using the Ab-Ag test, compared to antibody testing alone. The test uses a reaction known as "chemiluminescence" to detect antibodies and p24 protein antigen. In other words, if either the antibody or the antigen is present, the test reaction emits light that registers on a detector. There is only one currently approved antibody-antigen test, the Architect HIV Ag/Ab Combo assay. If this test is positive, it is recommended it be repeated. Tests that remain positive are confirmed with Western blot as described above.