ELISA Test
There are different ways to diagnose HIV, depending on the age of the person. When HIV gets into the body, the immune system makes a substance called an HIV antibody. HIV infection can be detected by an HIV antibody test, which is used for children over 12 to 18 months of age, adolescents, and adults in the U.S. This antibody test is called an enzyme-linked immunosorbent assay (ELISA) — a laboratory test used to detect the presence of antibodies in the blood. The ELISA is used to screen for HIV infection and a positive result indicates antibodies are present.

False Positive
After the ELISA test, a second or confirmatory test is necessary to rule out a "false-positive" test. The second test may be another ELISA, a rapid test (third generation ELISA), or a Western blot. A rapid HIV test is also an antibody test. The advantage of a rapid test is patients do not have to return at another time to get their test results — the results are usually available in approximately 30 minutes. Rapid tests are single-use and do not require laboratory facilities or highly trained staff, which makes them particularly suitable for use in developing countries.

The amount of time it takes for a person to develop antibodies after becoming infected with HIV varies. The majority of people with HIV will produce antibodies by about 45 days after infection. In a small proportion of people, it may take up to six months for antibodies to develop, and it may take even longer for a very few people. This is one reason why a lack of HIV antibodies does not always mean a person is free from infection, and repeat testing may be necessary.

Confirmed Positive
A confirmed positive test indicates a person has been infected with HIV. It is important to bear in mind that the HIV antibody test is not an "AIDS test"; there is no such thing.

Babies born to women infected with HIV will test positive for the HIV antibody at birth. This is because the mother’s antibody to HIV is passed to the baby in the womb. These babies are considered to be HIV-exposed. A newborn with HIV antibodies is not necessarily infected with HIV.

Other Tests
There are also tests that can look for the virus itself or parts of the virus (antigen testing and RNA viral load testing), damage to the immune system, or other aspects of the body's response to the effects of the virus. The best way to diagnose HIV infection in an infant younger than 12 to 18 months is to look for the virus itself in the infant’s blood. The most common test in the U.S. is HIV DNA PCR, which looks for the DNA (genetic material) of the virus. PCR can identify the virus in more than 90% of infants infected before birth and during delivery by the time they are one month of age, and in almost all infants infected before or during delivery by the time they are three to six months of age.

Testing has been shown to be part of an effective means of preventing HIV transmission. People who are aware of their HIV status are in a position to gain access to treatment, counseling, and education to prevent further transmission.

More information:
• Definition
• Symptoms
• Treatment
• Transmission