Which test would give the least reliable results in a 2-month-old infant?

• A. ELISA for HIV antibody
• B. CD4 T-cell count
• C. PCR for HIV proviral DNA
• D. p24 antigen

Answer: (A)

Maternal antibodies crossing the placenta can make antibody-based tests in infants unreliable. An ELISA for HIV antibodies detects antibodies, not the virus itself. In a 2-month-old, the infant may have maternal anti-HIV IgG circulating, so the ELISA result could reflect those maternal antibodies rather than the infant’s own infection status. That’s why this test is least reliable for diagnosing HIV in a young infant.

Tests that detect the virus or its components—such as PCR for HIV proviral DNA, which looks for viral genetic material, or p24 antigen detection, which identifies a viral protein—provide evidence of infection independent of maternal antibodies and are preferred for early diagnosis in this age group. CD4 T-cell count varies with age and doesn’t diagnose infection, so it’s not a reliable stand-alone test for confirming HIV in infants.