A short overview of how tests detect HIV antibodies.
Many people understand the importance of HIV-testing, but few understand how these tests actually work. Testing of HIV can be done by detecting antibodies against the virus, antigens specific to the virus, or by detecting viral DNA or RNA. The following article will examine how tests using antibody detection work.
When an individual is infected with HIV, that person mounts an immune response which culminates in the production of antibodies specific to the virus. The antibody can be detected using an enzyme-linked immunosorbent assay (ELISA). In this test, the HIV antigen to which the antibodies bind, needs to be available in a pure form. These antigens are attached to an enzyme which will become activated upon binding of the antigen to the antibody and will convert a colorless substrate to a colored product. The antibodies to be tested are attached non-specifically to plastic wells. The antigen-enzyme is washed through and if the antibodies are present, the antigen-antibody will couple causing a change in color. Non-specific binding of the antigen to the well is prevented by adding proteins that will not bind the antigen.
Antibodies are found throughout the fluids of the body. Therefore, samples of blood, urine, or saliva can be used to test for HIV antibodies.
The oral rapid testing uses the principals of ELISA. In this test, the individual uses a test strip to swab the inside of his or her cheeks and gums. Adhering to the test strip are HIV antigens. The antibodies in the oral mucosal transudate bind to the antigens on the strip. The end of the strip is then placed in an enzyme solution which travels up the strip. Upon binding to the antigen-antibody complex, the enzyme is activated and the strip changes color indicating a positive response. This test takes between 20 and 40 minutes to complete.
Because of the short lag time between completing the test and receiving the results, there is little chance that an individual will not receive his/her results as can happen when there is a lag time of days. Costs also decrease considerably. The tests themselves are cheaper and there is only one counseling session surrounding the actual test. Instead of providing counseling on two occasions, one during the actual test and another to go over the results, this information is shared during one visit. Also, there is much to be said for the decrease in psychological stress that can result in waiting days for test results. For a more complete overview of rapid HIV tests, please see Greenwald et al’s “ A Rapid Review of Rapid HIV Antibody Tests”.
