Early blood sampling in neonatal programs for the detection of phenylketonuria

A prospective study of blood phenylalanine levels during the early days of life of infants in 11 families with classical phenylketonuria (PKU) and 2 families with variant hyperphenylalaninemia has been performed. Four affected children were detected. In 3 infants with PKU, blood specimens obtained on the second day demonstrated levels of 8--10 mg% (480--600 micrometers/l) phenylalanine. In one infant with a variant type of abnormality (high phenylalanine requirement) only slightly elevated levels of 2--4 mg% (120--240 micrometers/l) were observed on the second to fourth days, though later very high levels were reached, warranting dietary treatment. In the routine neonatal screening program several variant cases were detected with borderline blood phenylalanine levels (2--4 mg%) in samples obtained on the third or fourth day of life. These results suggest that though probably very few cases of classical PKU might be missed by examination of blood samples obtained on the second postnatal day, the findings in variant cases justify a recommendation to obtain repeat blood specimens under such circumstances.