After delivery, a mother asks the nurse about newborn screening tests. The nurse explains that what is the optimal time for testing for phenylketonuria?

Phenylketonuria (PKU) screening requires that the newborn have ingested an adequate amount of protein — specifically the amino acid phenylalanine — for the test to yield a meaningful result. Phenylalanine accumulates in the blood only after the infant has been feeding, because the metabolic defect involves the inability to convert phenylalanine to tyrosine. Testing immediately after birth would produce a false-negative result because the infant has not yet consumed enough phenylalanine-containing milk to raise blood levels. Waiting until 2 to 3 days of age, after the infant has been feeding on breast milk or formula for at least 24 to 48 hours, allows sufficient phenylalanine accumulation to make screening reliable. Most hospitals screen before discharge, but if the infant is discharged before 24 hours, a repeat test is recommended at 1 to 2 weeks. Testing at 4 to 6 weeks or 2 months would be too late for early dietary intervention; PKU damage is largely preventable if a low-phenylalanine diet is initiated very early in infancy, ideally within the first few weeks of life. Delayed diagnosis allows toxic phenylalanine levels to cause irreversible intellectual disability and neurological damage. Therefore, 2 to 3 days after birth — following adequate milk intake — is the optimal and accepted timing for newborn PKU screening.