The human immunodeficiency virus -- HIV -- causes acquired immunodeficiency syndrome, or AIDS. You can be infected with HIV for many years and show few if any symptoms before the development of AIDS. However, people who are HIV-infected can transmit the virus to other people. Authors of a study published in 2005 in the "Journal of Infectious Disease" reported that the highest rate of HIV transmission occurs when an individual is newly infected but does not test positive for HIV.
The "window period" in HIV infection is defined as the time between the initial infection with the virus and a positive test for HIV. The length of the window period is different depending on the type of test used. A person is highly infectious during the window period because the HIV viral count becomes very high. The viral count, or number of viruses in the blood, is elevated because the immune system has not developed an immune response against HIV. Because most screening tests detect the antibodies made against the HIV virus -- which take time to produce -- repeat testing is recommended if initial testing is negative after a suspected exposure to HIV.
Antibody Screening Tests for HIV
HIV tests either detect antibodies that the infected individual has made against the HIV virus or detect the virus itself. Most initial HIV testing is done with a laboratory test that detects HIV antibodies. In addition to the antibodies, some tests may also detect a protein from the HIV virus called the p24 antigen. It takes approximately 25 days for an individual's immune system to detect the HIV virus and make antibodies against it. During the time before antibodies are made, an HIV antibody test will be negative even though the virus is present. The detection of antibodies to HIV can range from roughly 17 to 29 days, according to the Centers for Disease Control and Prevention.
HIV RNA Testing
Some HIV tests directly detect the genetic material, or RNA, of the virus. As reported by the New York State Department of Health AIDS Institute, during the initial phase of HIV infection, the amount of HIV RNA present typically ranges from 100,000 to more than 10 million copies of RNA per milliliter of blood. RNA from the HIV virus can be detected as soon as 6 to 12 days after exposure to the virus and about 2 weeks before antibodies are made. Tests that detect HIV RNA are very sensitive, however, and more costly to perform, and they are not typically used as screening tests.
New Testing Protocol That Improves Early Detection
Based on a 2013 report from CDC, a new testing algorithm to improve the detection of early HIV infection has been proposed. This is important to prevent the spread of HIV during a period when an individual may not know he is infected and could test negative for the virus. The new testing program uses screening and confirmatory antibody tests as well as HIV RNA testing in cases where the confirmatory antibody test is negative. A negative confirmatory antibody test is relatively common in the first weeks of an HIV infection. The addition of RNA testing will aid in reducing false negative HIV tests.
- Journal of Infectious Disease: Rates of HIV-1 Transmission per Coital Act, by Stage of HIV-1 Infection, in Rakai, Uganda
- Centers for Disease Control and Prevention: Advantages and Disadvantages of Different Types of FDA-approved HIV Immunoassays Used for Screening by Generation and Platform
- New York State Department of Health AIDS Institute: Diagnostic, Monitoring, and Resistance Laboratory Tests for HIV
- Centers for Disease Control and Prevention: Detection of Acute HIV Infection in Two Evaluations of a New HIV Diagnostic Testing Algorithm — United States, 2011–2013