A screening test for phenylketonuria (PKU) is performed on umbilical cord blood from a fair-skinned blond, blue-eyed infant born to dark-complexioned parents. The test is repoed as negative, and no dietary restrictions are imposed. At 1 year of age, the child is seen again, this time with obvious signs of severe mental retardation, and a diagnosis of PKU is made. The diagnosis was missed at bih because

The concentration of phenylalanine in affected infants is usually normal at bih and increases rapidly during the first days of life. False-negative results are common immediately after bih but are rare on the second and third days of life. Consequently, the blood sample for phenylketonuria is usually taken from the infant's heel within 2 to 3 days after bih. If the test is performed too early, the diagnosis could be missed.