HIV antibody and HIV antigen (p24) testing is used to screen for and diagnose HIV infections. Early detection and treatment of HIV infection and immune system monitoring can greatly improve long-term health and survival. Also, if a person knows his or her HIV status, it may help change behaviors that can put him or her and others at risk. Screening Different types of tests may be used for HIV screening: Combination HIV antibody and HIV antigen testûthis is the recommended screening test for HIV. It is available only as a blood test. It detects the HIV antigen called p24 plus antibodies to HIV-1 and HIV-2. (HIV-1 is the most common type found in the United States, while HIV-2 has a higher prevalence in parts of Africa.) The level of p24 antigen and the amount of virus (viral load) increase significantly soon after initial infection. Testing for p24 allows for detection of early infections, before HIV antibody is produced. A few weeks after exposure, antibodies to HIV are produced in response to the infection and remain detectable in the blood thereafter, making the antibody test useful for detecting infections weeks after exposure. By detecting both antibody and antigen, the combination test increases the likelihood that an infection is detected soon after exposure. These tests can detect HIV infections in most people by 2-6 weeks after exposure. HIV antibody testingûall HIV antibody tests used in the U.S. detect HIV-1 and some tests have been developed that can also detect HIV-2. These tests are available as blood tests or tests of oral fluid. HIV antibody tests can detect infections in most people 3-12 weeks after exposure. p24 antigen testingûthis is used alone without the antibody test only in rare cases when there is a question about interference with an HIV antibody test. There are a few different ways a person can get access to HIV screening: A blood or oral sample can be collected in a health practitioner's office or a local clinic and sent to a laboratory for testing. Certain testing centers provide either anonymous (the name is never given) or confidential (the name is given but kept private) HIV testing and counseling. People can also contact their state, county, or city health department to find out where testing may be available. To find a testing site near you, visit the National HIV and STD Testing Resources (link is external) webpage. In these same settings, there may be a rapid test available, with which results are generated in 20 minutes or less. A home collection kit is available that allows a person to take a sample at home and then mail it to a testing center. Results are available over the phone, along with appropriate counseling. There is a home test for HIV that uses an oral sample and results are available in about 20 minutes. The home test has two limitations: 1) testing on oral fluid is less sensitive than a blood test so the home test may miss some cases of HIV that a blood test would detect; and 2) the home test is not as accurate when it is performed at home by a lay person compared to when it is performed by a trained healthcare professional. However, the convenience of home testing might encourage some people who might otherwise be reluctant to go to a healthcare practitioner or clinic to learn their HIV status. Diagnosis If any one of the above screening tests is positive, then it must be followed by a second test to establish a diagnosis. This second test is an antibody test that is different than the first test. If the second test does not agree with the first test, then a third test is performed that detects the genetic material (RNA) of the virus. An HIV RNA test will detect HIV in most people by 1-4 weeks of infection.