Children get their antibodies for HIV from their mothers who are infected. If the child has antibody against HIV, it does not mean that the child is infected. Only 25 percent of children born to mothers with HIV actually are infected. 75 percent are carrying the antibody from the mother because the mother's blood goes from the mother to the baby during the pregnancy. As time passes, the HIV antibody dissipates and the HIV antibody disappears by the time the child is 15-18 months.
Some children lose the antibodies quite early in the first few months of life.
This can be done at any age. The test comes back in about 7-10 days. We then wait a month and repeat the PCR for HIV test. If the PCR is negative twice over a few months, then we consider the child uninfected as long as there are not other signs of infection. If the baby is growing and thriving and has no other signs of HIV infection along with two PCR tests being negative, then the child is considered to not be infected.