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Phenylketonuria (PKU) - Topic Overview

What is phenylketonuria (PKU)?

Phenylketonuria (PKU) is a rare genetic disorder in which the body cannot break down an amino acid called phenylalanine (say "fehn-uhl-AL-uh-neen"), which is a part of protein. This substance is found in breast milk, many types of baby formula, and most foods, especially those with a lot of protein, such as meat, eggs, and dairy products. If PKU is not treated, phenylalanine can build up in the blood and lead to intellectual disability and problems with the central nervous system (brain and spinal cord).

The good news is that early treatment can prevent all or most problems. Babies born with PKU need to start treatment with special formula soon after birth.

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PKU is more common in whites and Native Americans than in blacks, Hispanics, and Asians.

What causes PKU?

PKU is passed down through families. For a baby to have the disease, he or she must get (inherit) the PKU gene from both parents. The father and mother may not have PKU or even know that PKU runs in their families. If a baby gets the gene from only one parent, he or she is a carrier of the PKU gene but does not have the disease.

If you have a child with PKU and are thinking about having another baby, you may want to get genetic counseling. If you have a family history of PKU, talk with your doctor about genetic testing if you want to find out whether you carry the gene.

What are the symptoms?

If PKU is not found and treated soon after birth, symptoms usually start to appear within a few months after birth. (It takes time for the phenylalanine to build up in the baby's body.)

Early symptoms of PKU in a baby may include:

Without early treatment, the child may have growth problems, developmental delays, seizures, and severe intellectual disability.

How is PKU diagnosed?

Screening is recommended for all newborns within a few days after birth.1 If the PKU screening test shows that your baby has a phenylalanine problem, the doctor will do further testing to check whether your baby has PKU.

Finding and starting treatment for PKU early usually can prevent brain damage and other long-term problems.

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