DIAGNOSIS AND TREATMENT: INTERPRETATION OF RESULTS OF BLOOD SCREENING STUDIES FOR DETECTION OF PHENYLKETONURIA

THE American Academy of Pediatrics Committee on Fetus and Newborn recently recommended that a blood test for elevated concentration of phenylalanine be performed for all newborn infants prior to discharge. As results become available from wide application of blood screening tests, a number of questions have arisen regarding the interpretation of positive findings of elevated blood phenylalanine levels. Some reports suggest that diagnosis of phenylketonuria is determined by elevation of blood phenylalanine. The conclusion may be erroneously drawn that such a finding in an infant calls for immediate treatment with a low-phenylalanine diet. There are instances in which children were fed a phenylalanine restricted diet for periods up to 18 months and subsequently were proven not to have phenylketonuria. In another study plasma phenylalanine levels up to 40 mg/100 ml in a premature infant, accompanied by elevation of tyrosine, were shown not to result from phenylketonuria. Some investigators urge that tests for urinary metabolites characteristic of phenylketonuria be a necessary part of the diagnosis, while others consider urine testing of little value. The cause for the mental defect in phenylketonuria has not been established with certainty. No satisfactory explanations have been proposed to account for the rare individuals with normal intelligence and biochemical phenylketonuria. It is not known whether elevation of phenylalanine from disorders other than phenylketonuria might result in mental impairment. We have carried on a broad-scale urinescreening program since 1958 and a bloodscreening program since 1961. At the same time we have accumulated 70 patient years of experience in treatment of phenylketonuria.