Which test is most useful for diagnosing a vertically transmitted HIV infection in a newborn?

HIV PCR (Polymerase Chain Reaction) is the most useful test for diagnosing vertically transmitted HIV infection in a newborn. This test detects the presence of viral RNA or DNA in the blood, which is crucial for identifying HIV in infants, as maternal antibodies can interfere with other testing methods.

In the case of a newborn, HIV antibodies from the mother can persist for several months after birth, making serological tests like rapid HIV antibody tests or HIV IgM antibody tests unreliable for diagnosing infection in this context. Since PCR directly identifies the virus itself rather than the antibodies produced in response to the infection, it provides a more definitive diagnosis. A CD4 count is used to measure immune system health in individuals already diagnosed with HIV rather than to diagnose the infection itself, making it less relevant for this specific situation. Therefore, HIV PCR stands out as the most accurate and timely method for diagnosing HIV in neonates exposed to the virus.