Before the 1960s, most infants born with PKU developed mental retardation and cerebral palsy. Although treatment for PKU using a low pheny-lalanine diet was first described in the 1950s, the inability to detect PKU early in the child's life limited effective treatment. The first newborn screening test was developed in 1959 specifically to test for PKU. This simple yet effective and economical test was developed to screen newborn infants for PKU before they leave the hospital. Today all states routinely screen newborns for PKU. The American Academy of Pediatrics recommends that infants receiving the test during the first 24 hours of life be retested at two to three weeks of age during their first postnatal pediatric visit.

To test for PKU, a few drops of blood are taken from the infant's heel and then tested in a state laboratory for abnormal amounts of phenylalanine. The normal phenylalanine level is less than two milligrams per deciliter (mg/dl). Those with phenylalanine levels of 20.0 mg/dl or higher are considered likely to have "classical" PKU. Infants with these high levels are further tested to confirm the diagnosis before treatment is started.

Some infants with slightly higher levels of phenylalanine have "mild hyperphenylalanemia." Today many clinicians believe that any child with a phenylalanine level greater than six or eight mg/dl should be treated with a modified phenylalanine restricted diet.

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