Phenylketonuria Test

This is a blood or urine test used to determine if there is a deficiency of the enzyme called phenylalanine hydroxylase, which is an enzyme that converts phenylalanine to tyrosine (two amino acids that are necessary for growth). Phenylalanine must be broken down to tyrosine. When the enzyme to do this is not available, the phenylalanine accumulates in the body and spills over into the urine. When this becomes progressive, there is damage to the nervous system and delayed development can result. This test is not valid until after about 23 days and a newborn has ingested (taken in) an ample amount of phenylalanine. Phenylalanine is found in both human and cow's milk.


No preparation or fasting is necessary.


  • Negative less than or equal to 2 mg/dl (Guthrie technique)

  • Less than or equal to 121 μmol/L (SI units)

  • Urine: no green coloration.

Ranges for normal findings may vary among different laboratories and hospitals. You should always check with your doctor after having lab work or other tests done to discuss the meaning of your test results and whether your values are considered within normal limits.


Your caregiver will go over the test results with you and discuss the importance and meaning of your results, as well as treatment options and the need for additional tests if necessary.


It is your responsibility to obtain your test results. Ask the lab or department performing the test when and how you will get your results.