What is a PKU screening test?

A PKU screening test is a blood test given to newborns one to three days after birth. PKU stands for phenylketonuria. It is a rare disorder that prevents the body from breaking down part of a protein called phenylalanine (Phe). Phe is in all foods that contain protein, such as milk, meats, and nuts. It's also in an artificial sweetener called aspartame.

If you have PKU and eat foods with Phe, the Phe will build up in your blood. If the level gets too high, it can permanently damage your nervous system and brain. The damage can cause many types of health problems, including seizures, psychiatric problems, and learning and developmental disabilities. A PKU screening test diagnoses PKU by measuring the amount of Phe in a blood sample.

PKU is a genetic disorder. That means it's caused by changes (also called variants or mutations) in a gene. Genes are parts of DNA in your cells that you inherit from your parents. Genes carry information that controls what you look like and how your body works. The gene involved in PKU helps make a substance that breaks down Phe in food. PKU can vary from mild to severe, depending on whether the changed gene still works somewhat or has stopped working completely.

PKU is rare. For a child to inherit PKU, both parents must pass down a changed gene. But all newborns in the United States are required to get a PKU test. That's because:

• The test is easy and has almost no health risks. And doing the test can save a baby from lifelong brain damage and/or other serious health problems.
• If PKU is found early, a special diet that's low in protein and Phe can prevent health problems

Other names: PKU newborn screening, PKU test

What is it used for?

A PKU test is used to see if a newborn has high blood levels of Phe in their blood. This may mean the baby has PKU. More tests may be needed to confirm or rule out a diagnosis.

Why does my baby need a PKU screening test?

Newborns in the United States are required to get a PKU test. A PKU test is usually part of a series of tests called a newborn screening. Some older infants and children may need testing if they were born in another country. They may also be tested if they have any symptoms of PKU, which include:

• Delayed development
• Intellectual difficulties
• A musty odor in the breath, skin, and/or urine (pee)
• Unusually small head (microcephaly)

What happens during a PKU screening test?

A health care provider will clean your baby's heel with alcohol. Then the provider will stick the baby's heel with a small needle to collect a few drops of blood. The heel will then be bandaged.

To get accurate test results, the baby needs to have some protein from breast milk or formula before the blood sample is taken. That's why the test is usually done a day or two after birth. If your baby wasn't born in the hospital or if you left the hospital early, be sure to talk with your child's provider to schedule a PKU test as soon as possible.

Will I need to do anything to prepare my baby for the test?

There are no special preparations needed for a PKU test.

Are there any risks to the test?

There is very little risk to your baby from a needle stick blood test. Your baby may feel a little pinch when the heel is poked. A small bruise may form on the heel, but it should go away quickly.

What do the results mean?

If the results are normal, it means that the amount of Phe in your baby's blood was normal. So, your baby is unlikely to have PKU. But if the test was done sooner than 24 hours after birth, your baby may need to be tested again at 1 to 2 weeks of age.

If both parents know they carry a gene for PKU and baby's test is normal, the baby should have genetic testing. This blood test will check the baby's genes for changes. The results will diagnose PKU or rule it out for certain.

If your baby's results are not normal, it means high levels of Phe were found in your baby's blood. Your baby most likely has PKU. Your provider may order more tests to confirm or rule out PKU. These tests may include more blood tests and/or urine tests.

Because PKU is an inherited condition, you and your baby may have genetic testing. Genetic testing shows the type of gene change that's causing the disease. This information helps your provider understand how serious PKU is and how to treat it.

In general, PKU is treated with a special low-protein diet which must be followed throughout life. This usually means limiting or avoiding high-protein foods, such as meats, fish, eggs, and beans. People with PKU often take special food supplements for proper nutrition. In certain cases, PKU may also be treated with medicines. A nutrition specialist can help you plan a healthy diet to manage PKU.

If your baby is diagnosed with PKU, you can safely give the baby formula that doesn't contain Phe. Breastmilk does contain Phe, so if you would like to breastfeed, talk with your baby's provider to find out if your baby can safely have any amount of breastmilk.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a PKU screening test?

If you're pregnant, your baby could have PKU even if you and the baby's other parent don't have it. That's because you have two copies of all your genes, one from each parent. To have PKU, you must have changes in both copies of the gene that causes PKU.

If you and your baby's other parent each have one changed gene for PKU, your baby could inherit two changed genes and be born with the disease.

A prenatal screening test can show whether your baby may have PKU. The test uses either a sample of:

• Amniotic fluid that surrounds the baby in the uterus. A small amount of fluid is removed with a needle inserted into the belly during a procedure called amniocentesis.
• Tissue from part of the placenta, the organ that grows in the uterus to provide nutrients and oxygen to the unborn baby. This sample is taken using a needle inserted into the belly or a tube inserted through the vagina in a procedure called chorionic villus sampling.

Your provider or a genetic counselor can help you decide if prenatal testing for PKU is right for you.