The time between when a person may have been exposed to HIV and when a test can tell for sure whether they have HIV is called the window period. The window period varies from person to person and depends on the type of test used to detect HIV. A nucleic acid test (NAT) can usually tell you if you are infected with HIV 10 to 33 days after an exposure. An antigen/antibody test performed by a laboratory on blood from a vein can usually detect HIV infection 18 to 45 days after an exposure. Antigen/ antibody tests done with blood from a finger prick can take longer to detect HIV (18 to 90 days after an exposure). When the goal is to tell for sure that a person does not have HIV, an antigen/antibody test performed by a laboratory on blood from a vein is preferred. Antibody tests can usually take 23 to 90 days to reliably detect HIV infection. Most rapid tests and home tests are antibody tests. In general, antibody tests that use blood from a vein can detect HIV sooner after infection than tests done with blood from a finger prick or with oral fluid. Ask your health care provider about the window period for the test you’re taking. If you’re using a home test, you can get that information from the materials included in the test’s package. If you get an HIV test after a potential HIV exposure and the result is negative, get tested again after the window period for the test you’re taking to be sure. If your health care provider uses an antigen/antibody test performed by a laboratory on blood from a vein you should get tested again 45 days after your most recent exposure. For other tests, you should test again at least 90 days after your most recent exposure to tell for sure if you have HIV. If you learned you were HIV-negative the last time you were tested, you can only be sure you’re still negative if you haven’t had a potential HIV exposure since your last test. If you’re sexually active, continue to take actions to prevent HIV, like using condoms the right way every time you have sex and taking medicines to prevent HIV if you’re at high risk. Learn the right way to use a male condom and female condom. Learn more about how to protect yourself, and get information tailored to meet your needs from CDC’s HIV Risk Reduction Tool (BETA)