- Phenylketonuria Test (PKU Test, Guthrie Test, Phenylalanine): Phenylalanine hydroxylase is an enzyme which converts phenylalanine to tyrosine.
- A deficiency of this enzyme leads to a buildup of phenylalanine which results in severe mental retardation. This condition, known as phenylketonuria (PKU), is an autosomal recessive inborn error of metabolism.
- Screening of all newborns for PKU is required in all states. Testing is done either on the serum (Guthrie test) or on the
urine. Testing is not valid until the newborn has ingested an ample amount of the amino acid phenylalanine, which is found in human and cow’s milk. Two or three days of intake are usually sufficient for the Guthrie test.
- Urine PKU testing is usually done after the infant is 4 to 6 weeks old.