Diagnosing HIV Infections and AIDS

Every pregnant woman should be tested for HIV to have a better chance of preventing transmission to her unborn child.

If a woman knows she is HIV-infected and already has children, it is recommended that all of her children be tested for HIV. Even if she has older children and they seem healthy, they could still have an HIV infection if she was HIV-positive at the time they were born. A blood test is needed to know for sure.

However, when a new baby is born to an HIV-infected mother, there is no immediate way to know whether the baby is infected with the virus. This is because if the mother is infected, an ELISA test (which checks for HIV antibodies in the blood) will almost always be positive, too. Babies will have their HIV-infected mother's antibodies (which are passed to the baby through the placenta) even if they are not truly infected with HIV. These babies may remain HIV-antibody positive for up to 18 months after birth, even if they are not actually infected.

Infants who are not actually infected with the virus (but are born to HIV-positive mothers) will not make their own antibodies; the HIV antibodies that came from their mothers will gradually disappear from their blood before they reach 2 years of age. Any blood tests performed after this point will likely be HIV-negative. Infants who are infected with HIV from their mothers will begin to make their own HIV antibodies and will generally remain HIV-positive after 18 months of age.

The most accurate diagnosis of HIV infection in early infancy comes from tests that show the presence of the virus itself (not HIV antibodies) in the body. These tests include an HIV viral culture and PCR (polymerase chain reaction), a blood test that looks for the DNA of the virus.

Older children, teens, and adults are tested for HIV infection by an ELISA test to detect the presence of HIV antibodies in the blood. Antibodies are specific proteins that the body produces to fight infections; HIV-specific antibodies are produced in response to infection with HIV. Someone with antibodies against HIV is said to be HIV-positive. If the ELISA test is positive, it is always confirmed by another test called a Western blot. If both of these tests are positive, the patient is almost certainly infected with HIV.