In older children (and adults) the ELISA or rapid screening tests are used to confirm the diagnosis of HIV infection. These tests detect antibodies against HIV in the blood and usually become positive between 2 to 6 weeks after the infection (i.e. after the window period for the screening tests). As maternal HIV antibodies cross the placenta to the infant, and can remain in the infant’s blood for up to 18 months after delivery, the screening tests can only diagnose HIV infection in an infant if the test remains positive after 18 months of age. A positive screening test in the infant before 18 months may simply indicate that there are still maternal antibodies to HIV in an infant who is not infected (i.e. HIV-exposed but not infected).
A positive ELISA or rapid test in a child younger than 18 months does not necessarily mean the child has HIV infection.
A PCR test (polymerase chain reaction) detects HIV DNA (part of the genetic code of HIV). If the PCR blood test is positive the infant is infected with HIV. As the PCR test may take up to 6 weeks after the time of infection to become positive (the window period for the PCR test), an HIV-infected infant may have a negative test during this time (a false negative test).
If the mother is HIV-positive and does not breastfeed, the PCR test should be done on the infant 6 weeks after delivery (at the time of the first immunisation). If the mother breastfeeds, the test should only be done 6 weeks after the last breastfeed. It is a great advantage to establish whether an HIV-exposed child is infected with HIV or not.
The vast majority of infants (more than 98%) infected with HIV before or during delivery will have a positive PCR test by 6 weeks. The remainder will be positive by 3 months.
A positive PCR test indicates that the child has HIV infection.
The ultrasensitive p24 antigen test is as reliable as the PCR at 6 weeks but is not widely available yet. The PCR HIV RNA test is also probably an accurate method of identifying HIV infection in children under 18 months.