How do health care providers diagnose phenylketonuria (PKU)?

Nearly all cases of PKU are diagnosed through a blood test done on newborns.¹

Newborn Screening for PKU

All 50 U.S. states and territories require that newborns get screened for PKU. In addition to the United States, many other countries routinely screen infants for PKU.¹

Before screening for PKU was possible, most infants with the disorder developed severe intellectual and developmental disabilities (IDDs). In the 1960s, researchers supported by the federal Children’s Bureau determined that a test for PKU given to newborns was safe and effective. Later, NICHD led research on the safety and effectiveness of a restricted diet to treat PKU. Since then, PKU has been almost completely eliminated as a cause of IDDs.²

How are newborns tested for PKU?

Health care providers conduct a PKU screening test using a few drops of blood from a newborn’s heel. The blood sample, which can be used to screen for other conditions as well, is tested in a laboratory to determine whether it has too much phenylalanine in it. The newborn screening test should be performed by the child’s pediatrician if the mother did not give birth in a hospital or is discharged from the hospital before the test is performed.

What if my newborn tests positive for PKU?

If your newborn’s screening test comes back positive for PKU, your child will need additional tests to confirm that they definitely have the disorder. It is very important to follow your health care providers’ instructions for further tests. These tests may be blood or urine tests that may show whether the child has PKU. If your child does have PKU, getting treatment quickly will help protect your child’s health.³

Your health care providers may also suggest genetic testing to look at the mutations in genes that cause PKU. This testing is not required to figure out whether your child has PKU, but it will help identify the specific type of genetic mutation causing the disorder. This information may be useful for determining the best treatment plan going forward.⁴

Screening for PKU Later in Life

In the United States, newborn screening identifies nearly all people born with PKU.¹ However, there are concerns that cases of PKU could be missed due to errors at any step of the screening process—specimen collection, laboratory procedures, treatment initiation, or clinical follow-up. Missed cases are considered to be extremely rare. Because of these rare cases, health professionals recommend PKU testing if a person of any age has developmental delays or an intellectual disability.⁵

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.⁶ 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.