The diagnosis of phenylketonuria: a report from the Collaborative Study of Children Treated for Phenylketonuria.

One hundred ninety-five infants who met diagnostic criteria for enrollment in the Collaborative Study of Children Treated for Phenylketonuria (PKU) underwent a standard three-day challenge with 180 mg/kg/day of phenylalanine for confirmation of diagnosis. A sustained rise in serum phenylalanine levels of greater than 20 mg/dl was observed in 166 infants (85.1%), compatible with the diagnosis of classical PKU. In the remaining 29 infants (14.9%), the serum phenylalanine concentration either did not reach 20 mg/dL or, having achieved this level, subsequently declined below this point by 72 hours. It was agreed that these 29 patients had variant PKU and they were dropped from the Collaborative Study. We recommend that all infants diagnosed as having classic PKU undergo a challenge to confirm the diagnosis and need for continued treatment.