What Do Doctors Look for in a Phenylketonuria Test?

What Do Doctors Look for in a Phenylketonuria Test?

When babies are born, they undergo a newborn screening that tests them for certain heritable health conditions. In order to do this screen, doctors take a small drop of blood from the newborn using a heel stick.

One of the conditions doctors test for is phenylketonuria (PKU), a rare metabolic disorder characterized by abnormally high levels of phenylalanine (an amino acid, one of the building blocks of proteins) in the body.

What does the PKU test look for?

The PKU test measures the levels of an enzyme in the baby’s blood. This enzyme is called phenylalanine hydroxylase (PAH). It functions to metabolize phenylalanine into another amino acid called tyrosine. In PKU patients, there is a mutation in the gene┬áthat provides instructions for making the PAH enzyme. This results in reduced enzyme activity and causes phenylalanine to build up to toxic levels in cells and tissues.

During gestation, the mother’s PAH enzyme protects the baby but following birth, the baby has to rely on their own PAH enzyme. If they don’t have enough functional enzyme, they have PKU.

It’s important to do newborn screening in the first few days of life so that, if the baby does have PKU, they can start treatment before symptoms of the disease (such as brain damage) occur.

What happens after a positive test result?

If your baby tests positive for PKU, the doctors will order additional blood and urine tests to confirm the results. The baby and both parents also may need to undergo genetic testing.

Since PKU is an inherited disease, it’s important to know whether other family members may be affected. Following genetic testing, you will be referred to a genetic counselor who can explain the results of the genetic tests, and discuss with you the possibility of any future children also inheriting PKU.

Is there a treatment?

PKU is a treatable condition if detected early. You need to keep your baby on a low-protein diet. If you are not breastfeeding, you will need to use specific formulas that do not contain phenylalanine.

As your child grows, they will need to maintain a diet low in protein. It will be important to meet with a dietitian to ensure that your child is getting all the nutrients he or she needs while maintaining a low-phenylalanine diet.


Last updated: April 1, 2020


Phenylketonuria News is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.

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