A blood sample can be used to test for the mutations that cause PKU.
Testing an Infant
A blood test that measures the phenylalanine in an infant’s blood is enough to help make a PKU diagnosis. Therefore, DNA testing is not necessary. However, if a child tests positive for PKU, health care providers may recommend genetic testing because identifying the type of mutation involved can help guide selection of the most appropriate treatment plan.
A DNA test also should be performed on a child if both parents are PKU carriers and the standard newborn blood test does not show the condition. The test will definitively indicate or rule out PKU, if the disease-causing mutations in the family have been identified.1
Testing During Pregnancy
A pregnant person can request a prenatal DNA test to learn whether their child will be born with PKU. To perform this test, a health care provider takes some cells, either through a needle inserted into the abdomen or with a small tube inserted into the vagina. A genetic counselor who understands the risks and benefits of genetic testing can help explain the choices available for testing.1 This discussion may be particularly useful for parents who already have one child with PKU, because they have a higher-than-average chance of conceiving another child with the disorder. The disease-causing mutations must have been identified before prenatal testing can be performed.
Testing Possible PKU Carriers
If a child is diagnosed with PKU, other family members may be more likely to conceive children who also will have PKU.1 The parents’ siblings and other close blood relatives should be told that the child has PKU so that they can decide whether they should have DNA testing as well.
Maternal PKU is the term used when a woman who has PKU becomes pregnant. Most children born to mothers with PKU do not have the disorder. But if a pregnant woman who has PKU does not strictly follow a low-phenylalanine diet, her child can develop serious problems. These include:
- Intellectual and developmental disabilities (IDDs)
- Having a head that is too small (microcephaly)
- Heart defects
- Low birth weight
- Behavioral problems2
The newborn’s problems from untreated maternal PKU are caused by the high phenylalanine levels present in the mother’s blood during pregnancy—not by PKU itself. The infant does not have PKU3 and does not need a PKU diet. The PKU diet will not help with these health problems.
Women with PKU and uncontrolled phenylalanine levels also have an increased risk of pregnancy loss.4
If you have PKU, it is very important to follow a strict low-phenylalanine diet before becoming pregnant and throughout your pregnancy.
In addition to staying on a PKU diet, also make sure to:
- Visit a PKU clinic on a regular basis
- Have your blood checked often for phenylalanine
- Ask your health care provider how much PKU formula to drink
Keep in mind that untreated maternal PKU can cause serious problems for a developing fetus.5
A newborn’s problems from untreated maternal PKU are caused by the high phenylalanine levels present in the mother’s blood during pregnancy—not by PKU itself. The infant does not have PKU4 and does not need a PKU diet. The PKU diet will not help with these health problems.
Women with PKU and uncontrolled phenylalanine levels also have an increased risk of pregnancy loss.4
A number of factors influence whether a person with PKU has mild symptoms or more severe problems.
Genetic Factors
Many different mutations of the PAH gene can cause PKU. The type of mutation greatly affects the severity of the person’s symptoms.
Some mutations cause classic PKU, the most severe form of the disorder. In these cases, the enzyme that breaks down phenylalanine barely works or does not work at all. If it is not treated, classic PKU can cause severe brain damage and other serious medical problems. Some mutations allow the enzyme to work a little better than it does in classic PKU. This is sometimes called non-PKU hyperphenylalaninemia, and is also known as non-PKU HPA. Such cases come with a smaller risk of brain damage. People with very mild cases may not require treatment with a low-phenylalanine diet.3
Non-Genetic Factors
Genes are not the only factor that influences the severity of PKU symptoms. For example, strictly following a PKU diet greatly reduces the chances that a person will have IDDs and other problems caused by PKU. Other factors include the person’s age at diagnosis and how quickly the person’s blood levels of phenylalanine are brought under control.6
Infants and children with PKU need frequent blood tests to measure the phenylalanine in their blood. Their health care provider may suggest changes to the diet or formula the child receives if there is evidence of too much or too little phenylalanine.
Infants with PKU will be tested about once a week for the first year of their lives. After the first year, children may be tested once or twice a month. Adults also need to be checked regularly throughout their lives.6 Often, blood samples can be taken at home and mailed to a laboratory.7
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