Newborns are routinely screened for phenylketonuria.

(A) The infant has not ingested any protein immediately after birth, which is necessary to detect excessive serum phenylalanine. (B) It is important that the infant take in 2–3 full days of milk or formula feedings to preclude a false-negative reading. (C) At 2–3 days of age, inadequate milk could have been ingested owing to a delay in the initial feeding. (D) The biochemical buildup of serum phenylalanine is detectable after 2–3 days of milk or formula ingestion.