A newborn screen for phenylketonuria (PKU) is collected from the baby’s blood. When should the sample be obtained?

• A. At birth from cord blood.
• B. Fourteen days after birth.
• C. Before oral feedings are initiated.
• D. After ingestion of a source of protein.

Answer: (D)

The test is designed to detect elevated phenylalanine levels, which reflect the infant’s ability to metabolize phenylalanine from dietary protein. If the sample is taken too early—at birth, from cord blood, or before the baby has fed—the maternal supply of phenylalanine and the newborn’s currently low intake can mask a defect, leading to a false-negative result. Waiting until after the infant has ingested protein ensures that phenylalanine appears in the blood if PKU is present, making the test reliable. Therefore, collecting the sample after ingestion of a protein source is the best timing.