What Is Genetic Testing for Phenylketonuria?

What Is Genetic Testing for Phenylketonuria?

The vast majority of babies have a phenylketonuria (PKU) test at birth to assess for the inherited disorder. In most countries in the world, this is part of newborn screening. However, this type of testing does not identify PKU carriers – people who will not develop the disease themselves but have the potential to pass it on to their children.

If your partner has PKU, or if the disease runs in either of your families, you may want to have a genetic test to find out whether you are a carrier of the disease and have a risk of having children with PKU. If you and your partner have already conceived, you may want to consider prenatal genetic testing to determine if your child may have PKU.

What is genetic testing?

Genetic testing is an assessment that determines if you have mutations, deletions, or chromosome changes that may cause a genetic disease. This type of testing can confirm or rule out a suspected genetic disease diagnosis.

The decision to have a genetic test is a complicated one. You should discuss with your primary care physician and a genetic counselor to determine whether testing is necessary and whether it will give you the answers you want.

Genetics of PKU

PKU is a genetic disease that is inherited in a recessive pattern. This means that a child must inherit two copies of a defective gene, one from each parent, to develop the disease.

The gene that is affected in PKU is called PAH. When there are mutations in this gene, patients are unable to metabolize the amino acid phenylalanine — found in all proteins and in some artificial sweeteners. Of note, amino acids are the building blocks of proteins. If not treated with a low-phenylalanine diet, the disease causes extremely high levels of this amino acid in the body. This can lead to irreversible intellectual disability and other health problems.

If you have PKU, and your partner is a carrier, then your children will have a 1-in-2 chance of inheriting two copies of the disease-causing mutation. If this occurs, your child will develop PKU. Your children also will have a 1-in-2 chance of being carriers.

If both you and your partner are carriers, your children will have a 1-in-4 chance of developing PKU. In turn, they also will have a 1-in-2 chance of being carriers. Finally, your children will have a 1-in-4 chance of inheriting no copies of the disease-causing mutation. That means that they will neither have PKU nor be a carrier of the disease.

How does genetic testing work?

Your doctor and genetic counselor will help you decide whether you should have a genetic test for PKU. Your doctor can collect a blood sample that is sent to a laboratory for genetic testing.

If you are considering prenatal genetic testing, you and your partner will have to determine how doctors will collect DNA from the fetus. There are two main techniques used: amniocentesis, in which amniotic fluid is removed from the mother’s uterus for testing,  and chorionic villus sampling (CVS), in which a sample from the placenta is removed for examination. Each has risks, and they can be performed at different stages of development. Your doctor will advise you on which method may suit you best.

What happens after the genetic test?

Genetic testing takes a few days to a few weeks to complete. When the results are back, your doctor and genetic counselor can review them with you and explain what they mean. If you or your child have a genetic mutation, your healthcare professionals will discuss any additional tests that may be necessary to confirm a diagnosis. They also will outline treatment options once the diagnosis is final.

What are the risks of genetic testing?

Having a genetic mutation doesn’t necessarily mean that you will have symptoms, or how severe they will be. Some people might prefer not to know that they have a genetic mutation unless or until they develop symptoms.

In prenatal genetic testing, the method of DNA collection has some risks to both the mother and the fetus. Although these risks are generally low, your doctor may wish not to test for PKU until after your child is born. This is because treatment will not be possible until after birth.


Last updated: July 23, 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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