Phenylketonuria Test

Why is my child having this test?

Phenylketonuria (PKU) is a disease in which the body lacks an enzyme that digests an amino acid called phenylalanine. Phenylalanine is most often found in foods that contain protein, such as meat, cow's milk, over-the-counter infant formula, and breast milk.

The PKU test is part of routine testing that babies have at the hospital after birth (newborn metabolic screening). The test is usually done within 1–2 days after birth, or after your baby has had enough milk to get an accurate test result. It is typically done for all newborns. However, it is especially important to have this test if the newborn has a family history of PKU because the disease is passed from parent to child.

What is being tested?

This test measures how much of an enzyme called phenylalanine hydroxylase is in the blood.

This enzyme is made in the liver. It converts the amino acid phenylalanine into a different amino acid called tyrosine. These amino acids are necessary for growth. However, too much phenylalanine can cause brain damage and intellectual disability. Having too little of the enzyme phenylalanine hydroxylase means that your child has too much phenylalanine in his or her body.

What kind of sample is taken?

A blood sample is required for this test. It is usually collected through a needle prick in the back of the foot (heel stick).

How are the results reported?

Your child's test results will be reported as either positive or negative for PKU. Positive means that there is too little phenylalanine hydroxylase in your child's blood, and negative means that there is a normal amount.

What do the results mean?

A negative test result means that your child does not have PKU. This is considered a normal result.

A positive test result means that your child may have PKU and will need more testing. He or she may need to eat a special diet that excludes phenylalanine.

Talk with your child's health care provider about what your child's results mean.

Questions to ask your child's health care provider

• When will my child's results be ready?
• How will I get my child's results?
• What are my child's treatment options?
• What other tests does my child need?
• What are my next steps?

Summary

• Phenylketonuria (PKU) is a disease that is caused by a lack of the liver enzyme that is needed to digest an amino acid called phenylalanine.
• The PKU test is part of routine testing that babies have at the hospital after birth (newborn metabolic screening).
• A negative test result means that your baby does not have PKU. A positive test result means that your baby may have PKU and will need more testing.
• Talk with your child's health care provider about what your child's results mean.