Many people who are infected with HIV do not know they have the disease and can potentially transmit it to others. Although roughly 16 to 22 million people get tested each year, about 1 in 5 (21%) Americans who tested positive did not know they had the disease, according to 2006 year-end statistics from the US Centers for Disease Control and Prevention (CDC). The CDC estimates that there were well over a million people in the United States living with HIV by the end of 2006 and there were more than 56,000 new infections. Effective screening and early detection allows for appropriate treatment and informs people of their status in order to help stop spread of the disease.
Most current tests used for HIV screening in clinics and other health care facilities detect HIV antibody. Such tests are of value when the affected person has had time to develop antibodies to HIV, which can take anywhere from 2 to 8 weeks. People may have to wait to be tested until weeks after a suspected exposure to avoid false-negative results. Tests that detect the p24 antigen, a protein produced by the virus, can detect HIV in blood before antibodies develop, enabling them to detect the infection in the earlier stages. The p24 antigen test can determine the presence of the virus from about one week until 3 to 4 weeks after initial infection. As the disease progresses and HIV antibodies begin to be produced, they bind to the p24 antigen, causing it to become undetectable in the blood. At that stage, antibody levels are typically high enough to be detected with the HIV antibody test.
Thus, the p24 test is most useful for the very early, acute stages of the infection, while tests for HIV antibody are positive and remain positive 2 to 8 weeks after infection. By combining these tests into a single assay, the newly approved test increases the probability of identifying those infected sooner after their exposure to HIV. However, there is a window period of about 1 week immediately following exposure, when virus levels are relatively low, and the p24 antigen cannot be detected. As with a positive result from an HIV antibody screening test, a positive result from the combination test will require follow-up tests in order to confirm a diagnosis.
A molecular test, known as HIV viral load or the nucleic acid amplification test (NAAT), is currently the most sensitive means of determining if someone is infected with HIV. However, this test is not used for screening purposes because it is not usually available or practical for use in clinic settings. The new combination antibody-antigen test represents a realistic means of improving screening detection rates of HIV in the general population.
The new test is not approved for testing donated blood, but it may be used where NAAT is not available or practical, according to the FDA. Of particular note, the test is approved for use in pregnant women and in children as young as 2 years old, two populations in which it is critical to receive prompt treatment for the best outcome.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
(October 27, 2009) Centers for Disease Control and Prevention. HIV Testing. Available online at http://www.cdc.gov/hiv/topics/testing/index.htm through http://www.cdc.gov. Accessed August 2010.
(June 21, 2010) Abbott Diagnostics Press Release: FDA approves first-of-its-kind HIV test which can detect HIV days earlier than current US tests. Available online at http://www.abbott.com/global/url/pressRelease/en_US/60.5:5/Press_Release_0875.htm?referrer=RSS through http://www.abbott.com. Accessed August 2010.