•All newborns receive a heel stick to test the serum for several metabolic disorders. The tandem mass spectrometry can measure the levels of several amino acids to confirm the presence of PKU.
••The Guthrie test was once the only method used to measure serum phenylalanine levels, but is frequently being replaced with tandem mass spectrometry.
•Liver enzyme analysis can be done to measure the activity of phenylalanine hydroxylase, but this is generally not done.
•Urinalysis can detect ketones in the urine of a child with PKU, however, this does not confirm the diagnosis.