The purpose of screening infants for the presence of phenylketonuria is to permit early treatment and thus reduce the risk of mental retardation associated with this disease. It has been assumed that the chief problem lay in detection of this rare disorder, and the difficulties and hazards of treatment have not been emphasized. Death as a result of overtreatment of phenylketonuria has occurred. Growth failure, anemia, and rachitic bone changes are also described in treated children, presumably the result of phenylalanine deficiency. Mild to moderate mental retardation has been reported in infants who were treated by severe restriction of phenylalanine intake in spite of early detection of the disease.
It should be recognized that the detection of phenylketonuria in an infant is but the first step in a long-term treatment program if mental retardation is to be prevented. The most critical period of treatment comes during the first six months
Helen K. Berry, Betty S. Sutherland, Barbara Umbarger. Detection of Phenylketonuria in Newborn Infants. JAMA. 1966;198(10):1114–1115. doi:10.1001/jama.1966.03110230130031