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Phenylketonuria (PKU): Other FAQs

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Basic information for topics, such as “What is it?” and “How many people are affected?” is available in the Condition Information section. In addition, Frequently Asked Questions (FAQs) that are specific to a certain topic are answered in this section.

Is genetic testing for PKU available?

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 woman can request a prenatal DNA test to learn whether or not her 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 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 or not they should have DNA testing as well.

What is maternal PKU?

Maternal PKU is the term used when a woman who has PKU becomes pregnant. Most children born to PKU mothers 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 disabilities
  • 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 these health problems.

Women with PKU and uncontrolled phenylalanine levels also have an increased risk of pregnancy loss.4

If I have PKU, what steps should I take during pregnancy to protect my infant?

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 PKU itself. The infant does not have PKU4 and does not need a PKU diet. The PKU diet will not help these health problems.

Women with PKU and uncontrolled phenylalanine levels also have an increased risk of pregnancy loss.4

What determines the severity of PKU?

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 intellectual disabilities 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

Does a child with PKU need repeated testing?

Infants and children with PKU need frequent blood tests to measure the phenylalanine in their blood. The 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

  1. Screening, Technology, and Research in Genetics. (2007). Amino acid disorders: Phenylketonuria [Fact sheet]. Retrieved May 15, 2012, from External Web Site Policy [top]
  2. National Human Genome Research Institute. (2010). Learning about phenylketonuria (PKU). Retrieved May 15, 2012, from [top]
  3. American Congress of Obstetricians and Gynecologists. (2009, December). Maternal Phenylketonuria. ACOG Committee Opinion Number 449. Retrieved June 22, 2012, from External Web Site Policy [top]
  4. Genetics Home Reference. (2012). Phenylketonuria. Retrieved May 15, 2012, from [top]
  5. National Human Genome Research Institute. (2006). Promoting safe and effective genetic testing in the United States: The problem of maternal PKU. Retrieved May 15, 2012, from [top]
  6. NICHD. (2000, updated 2006). Report of the NIH consensus development conference on phenylketonuria (PKU): Screening and management. Retrieved May 15, 2012, from [top]
  7. University of Washington, Seattle, Christine H. Trahms Program for Phenylketonuria. (2004). The essentials of phenylketonuria: For young adults with PKU and their significant others. Retrieved May 15, 2012, from External Web Site Policy [top]

Last Reviewed: 08/23/2013
Vision National Institutes of Health Home BOND National Institues of Health Home Home Storz Lab: Section on Environmental Gene Regulation Home Machner Lab: Unit on Microbial Pathogenesis Home Division of Intramural Population Health Research Home Bonifacino Lab: Section on Intracellular Protein Trafficking Home Lilly Lab: Section on Gamete Development Home Lippincott-Schwartz Lab: Section on Organelle Biology