While assessing a newborn, the nurse suspects the presence of phenylketonuria (PKU). Which of the following tests would confirm the diagnosis?


• 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.

• A normal PKU is less than 2 mg/dl.

• Phenylalanine is an amino acid normally present in the body. This test is done to check whether the baby has an enzyme needed to change phenylalanine into tyrosine. Without it, the phenylalanine level builds up in the body, causing brain damage, microcephaly, and cerebral impairment.


• The Guthrie test was once the only method used to measure serum phenylalanine levels. It has been 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, but this does not confirm the diagnosis.

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