Correlates of intelligence test results in treated phenylketonuric children

The Collaborative Study of Children Treated for Phenylketonuria was initiated in 1967. The data presented are based on findings in 132 children treated from near birth to 6 years of age. Two treatment groups were randomly formed. The target for blood phenylalanine was 1.0 to 5.4 mg/100 ml for group 1 and 5.5 to 9.9 mg/100 ml for group 2. Although mean blood phenylalanine levels for both groups initially fell within the prescribed ranges, a steady increase over time resulted in mean six year levels of 11.4 mg/100 ml and 13.0 mg/100 ml for the two groups, respectively. Because it was not possible to maintain the prescribed differences in blood phenylalanine levels between the two groups, they were combined for further analyses. The mean IQ of the total sample at age 6 years was 98 on the Stanford-Binet Intelligence Scale. Multiple regression analysis showed that, among selected treatment and psychosocial factors, the most important predictors of IQ for 6-year-old children were: (1) mothers' intellectual ability (as measured on the Wechsler Adult Intelligence Scale); (2) age at which the subjects were first treated; and (3) how well the subjects adhered to the phenylalanine-restricted diet. It was concluded that optimal early treatment will result in normal levels of intelligence at 6 years of age.