HIV testing remains an integral part of public health efforts to control the spread of HIV/AIDS and provide early treatment for people with the illness. Different types of HIV tests detect HIV antigens or antibodies. An HIV antigen is a component protein of the virus. An HIV antibody is a protein produced by the immune system to fight the virus. Most HIV tests detect HIV antibodies. HIV antigen tests, however, prove useful in certain circumstances.
EIA Antibody Tests
An EIA, or enzyme immunoassay, is the most commonly used laboratory test used to screen for HIV antibodies, according to the Centers for Disease Control and Prevention. A blood, urine or oral fluid sample can be used for an EIA test, also known as an ELISA test. Samples that test positive for HIV antibodies using EIA screening undergo further testing with another procedure to confirm the result.
An important limitation of EIA and other HIV antibody screening tests is the possibility of missing an early HIV infection. After contracting HIV, the immune system responds by producing antibodies. It takes approximately two to eight weeks, however, for HIV antibodies to reach a level detectable by laboratory screening methods, notes the CDC. Therefore, people screened for antibodies during the first several weeks after HIV infection may have a negative test result. Doctors often recommend repeat screening at a later date or another type of HIV testing for people with a known exposure to the virus and a negative EIA test result.
Rapid HIV Antibody Tests
Rapid HIV antibody tests utilize technology similar to an EIA in a self-contained testing unit that can be used in settings outside the traditional laboratory. Test results are usually available in approximately 20 minutes, reports the CDC. As with an EIA screening test, a positive rapid HIV test requires further testing to confirm the result. A negative rapid HIV test result may occur in a patient with HIV who does not yet have a detectable antibody level.
Western Blot
A Western blot is a highly specific test for HIV antibodies. The test is most commonly used to confirm the results of a positive EIA screening test, reports New York University Medical Center. Patients with a positive EIA screening test and a positive Western blot test have an HIV infection. A positive EIA test with a negative or indeterminate Western blot test necessitates further testing to determine whether the patient has HIV.
HIV p24 Antigen Test
The HIV p24 antigen test detects an HIV protein known as p24. Measurable amounts of the p24 protein often circulate in the bloodstream during the first few weeks after HIV infection, before an appreciable level of HIV antibodies develop. The p24 HIV antigen test is primarily used to screen donated blood for HIV. Doctors also sometimes order a p24 antigen test to screen for HIV in infants whose mothers have the illness, reports Neil Constantine, Ph.D., of the University of Maryland School of Medicine.
