The usual HIV test is one that detects antibodies to HIV in the blood. Rarely, a single HIV test for an individual person may not be reliable.The usual recommendation in diagnosing HIV infection is therefore to perform two tests. Both tests should be positive for a diagnosis of HIV infection.
A positive HIV antibody test is not a reliable indicator of HIV infection in infants. During the pregnancy of a woman with HIV infection, antibodies to HIV cross the placenta.Therefore almost all children born to HIV-positive mothers have HIV antibodies in their blood at birth. However, only about one-third of children born to HIV-infected mothers are infected. Initially, therefore, HIV antibody testing cannot distinguish uninfected from infected children. In uninfected children, these maternal antibodies usually become undetectable by 9 months of age, but occasionally they remain detectable until 15 months. Most infected children make their own antibodies, so the HIV antibody test will still be positive after 15 months.
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