What is the gold standard for testing HIV in infants from birth until 18 months?

The gold standard for testing HIV in infants from birth until 18 months is polymerase chain reaction (PCR). This is because infants can carry maternal antibodies for a period following birth, which can result in false positives if antibody-based tests, such as the ELISA or rapid antibody tests, are used.

PCR detects the actual viral RNA in the infant's blood, allowing for the diagnosis of HIV regardless of whether maternal antibodies are present. This also makes PCR capable of diagnosing HIV infection within the first few weeks of life, right when timely intervention is crucial for the infant's health.

Furthermore, PCR testing is particularly sensitive and specific for detecting HIV in this age group since it confirms the presence of the virus rather than relying on antibody response, which may not be present or may be confounded by maternal factors. Other methods like Western blot are used for confirming HIV in older children and adults, as they depend on the presence of antibodies to the virus, which would not be a reliable marker in infants.