1. What is the Window Period?
The window period is the length of time after infection that it takes for a person to develop enough specific antibodies to be detected by our current testing methods. If an individual engages in unsafe sex or shares drug injection equipment and becomes infected, the body will make antibodies to fight HIV. When enough antibodies are developed, the HIV antibody test will come back positive. Each person's body responds to HIV infection a little differently, so the window period varies slightly from person to person. HIV is most commonly diagnosed in adolescents and adults through HIV antibody testing. However, there are also tests that diagnose HIV infection by detecting certain parts of the genetic material of HIV. PCR tests (polymerase chain reaction, often the same tests used to monitor the ‘viral load’ response to HIV therapy) are used to diagnose HIV infection in infants and to make the diagnosis of acute HIV infection. These HIV PCR tests are used when the clinical suspicion is high for acute HIV infection, the standard screening antibody tests are negative, and the confirmatory Western blot tests are either negative or indeterminant. Viral culture may also be performed in certain circumstances to diagnose HIV, but this method is now rarely ever used.
2. How Has Our Understanding of the Window Period Changed Over the Years?
Early in the epidemic, our testing methods were not as sensitive as they are today. Doctors and public health officials wanted to make sure that people who engaged in risk behaviors for HIV were tested long enough after their risk to be sure that anyone who was actually infected would test positive. The Centers for Disease Control currently states that people with possible exposure to HIV, who test negative, should be re-tested 6 months after the possible exposure to ensure that sufficient time has elapsed to make antibodies. Improvements in HIV testing technology, increasing laboratory experience with testing and the ability to better monitor early infection through PCR testing have contributed greatly to our understanding of the window period and have provided increased confidence that virtually all cases of HIV infection can be detected by the standard antibody screening tests 3 months after the possible exposure.
3. What is the best time for the first HIV test after a possible exposure to HIV?
Most people infected with HIV will develop enough antibodies to be detected by our current HIV antibody tests 4 weeks after the exposure. This means that, for example, if a person had unsafe sex and became infected on January 1, it is likely that he/she will have enough antibodies to test positive 4 weeks later. If the person tests positive, this early testing is beneficial because the person can begin receiving medical care very early in the course of the infection. Recent advances in care and treatment for HIV infection have increased the advantages of early identification and treatment. Therefore, especially when HIV infection is highly suspected, the first HIV test should take place 4 weeks after the exposure. In cases of occupational exposure or exposure through sexual assault, an HIV test is also recommended immediately after the exposure to establish baseline HIV status. The protocol for testing following occupational exposure recommends testing at baseline, 1 month, 3 months, and 6 months post-exposure. See HIV Prophylaxis Following Occupational Exposure.
4. How long after a possible exposure to HIV does the person have to wait to be tested to be sure he/she is not infected?
It is possible that someone who tests negative 4 weeks after an exposure may be infected but his/her body has not had sufficient time to develop antibodies. Therefore, to rule out HIV infection, it is important to re-test 3 months after the exposure. If symptoms suggestive of HIV acute seroconversion syndrome (e.g., fever, fatigue or malaise, joint pain, headache, loss of appetite, rash, night sweats, myalgias, nausea or diarrhea, pharyngitis) develop, medical evaluation should occur immediately. A plasma HIV RNA assay should be used in conjunction with an HIV-1 antibody test to diagnose acute HIV infection. It is extremely rare for an HIV-infected individual to not develop antibodies by 3 months. An individual who tests negative 3 months after an exposure does not require further testing unless he/she may has had repeated exposures or if their antibody test results are incompatible with their clinical history.
5. What is the recommendation for testing individuals who engage in on-going risk behavior?
The primary focus of our work with individuals who place themselves at ongoing risk for HIV infection must be continued education, behavioral counseling and harm reduction, such as education about safer injection practices and referral to syringe exchange programs and drug rehabilitation services. HIV testing offers no "protection" from HIV infection. When an individual is engaging in ongoing risk behavior, it is not possible to develop a timeline for re-testing based on a single exposure. An individual with a negative HIV test who engages in on-going risk behavior should be offered testing every 3 months and counseled to avoid risk behavior. In these cases, the function of testing is to ensure early access to care in the event that the individual becomes HIV positive.