Because early HIV infection often causes no symptoms, a healthcare provider usually can diagnose it by testing blood for the presence of antibodies (disease-fighting proteins) to HIV. HIV antibodies generally do not reach noticeable levels in the blood for 1 to 3 months after infection. It may take the antibodies as long as 6 months to be produced in quantities large enough to show up in standard blood tests. Hence, to determine whether a person has been recently infected (acute infection), a healthcare provider can screen blood for the presence of HIV genetic material. Direct screening of HIV is extremely critical to prevent transmission of HIV from recently infected individuals.

Anyone who has been exposed to the virus should get an HIV test as soon as the immune system is likely to develop antibodies to the virus -- within 6 weeks to 12 months after possible exposure to the virus. By getting tested early, a healthcare provider can give advice to an infected person about when to start treatment to help the immune system combat HIV and help prevent the emergence of certain opportunistic infections (see section on treatment). Early testing also alerts an infected person to avoid high-risk behaviors that could spread the virus to others.

Most healthcare providers can do HIV testing and will usually offer counseling at the same time. Of course, testing can be done anonymously at many sites if a person is concerned about confidentiality.

Healthcare providers diagnose HIV infection by using two different types of antibody tests: ELISA (enzyme-linked immunosorbent assay) and Western blot. If a person is highly likely to be infected with HIV but has tested negative for both tests, a healthcare provider may request additional tests. A person also may be told to repeat antibody testing at a later date, when antibodies to HIV are more likely to have developed.

Diagnosis in Babies

Babies born to mothers infected with HIV may or may not be infected with the virus, but all carry their mothers' antibodies to HIV for several months. If these babies lack symptoms, healthcare providers cannot make a definitive diagnosis of HIV infection using standard antibody tests. Instead, they are using new technologies to detect HIV and more accurately determine HIV infection in infants between ages 3 months and 15 months. Researchers are evaluating a number of blood tests to determine which ones are best for diagnosing HIV infection in babies younger than 3 months.