PKU Test
A PKU screening test looks for high levels of the amino acid phenylalanine in your baby’s blood. This could mean that they have PKU, a genetic condition that causes phenylalanine to build up in their blood and brain, causing damage. The PKU test is part of newborn screening blood tests in the U.S.
What Is A PKU Test?
A PKU test is a type of newborn screening that looks for signs of phenylketonuria (PKU). PKU is a genetic condition (one you’re born with). It prevents your baby’s body from breaking down the amino acid phenylalanine (phe) that’s in certain foods. The amino acid builds up in your baby’s body and can damage their brain. This causes symptoms that include developmental delay, hyperactivity, seizures and intellectual disability.
Healthcare providers screen your baby for PKU within a few days of birth.
Why does my baby need a PKU test?
A PKU test is part of standard newborn screening in the U.S. Identifying the condition early means your baby can start treatment as soon as possible to prevent lifelong damage. Your child’s healthcare provider might also recommend this test if your child has symptoms of PKU or you have a family history of PKU.
How does a PKU test work?
Providers run a PKU test on a sample of blood. They’ll use a small needle to prick your baby’s heel to get a few drops of blood. Then, they’ll test it for high levels of phenylalanine.
How do I prepare for the test?
There’s nothing you or your baby needs to do to prepare for the test. But your baby does need to have had breastmilk or formula leading up to it. This is why it’s not done until a day or two after birth.
What are the risks of a PKU test?
There aren’t a lot of risks involved in a PKU test. It shouldn’t be very painful, but your baby might not be happy with getting their heel pricked. They might develop a small bruise or scab where blood was drawn.
If your baby has a bandage over the area, ask their healthcare provider when it’s OK to remove it. Remove it gently so you don’t open the scab again.
What kind of results do you get from a PKU test?
Your baby’s results might be marked as normal, abnormal, high or positive. Normal phe levels are below 2 mg/dL (milligrams per deciliter). A normal result means your baby doesn’t need further testing for PKU.
Abnormal or high levels are above 4 mg/dL. You’ll usually know the results of a PKU screening in about a week.
What does it mean if a PKU test is positive?
A positive PKU test means the results showed high levels of phe. But it doesn’t mean your baby has PKU. It means your baby needs follow-up testing to confirm a PKU diagnosis. They’ll get another blood test. Sometimes, follow-up testing shows normal levels of phe.
Waiting for the results might make you feel anxious. But PKU is manageable. If additional testing confirms PKU, your child’s healthcare provider might refer you to a specialist. They’ll work with you to manage the condition.
References
- Fabie NAV, Pappas KB, Feldman GL. The Current State of Newborn Screening in the United States (https://pubmed.ncbi.nlm.nih.gov/30819343/). Pediatr Clin North Am. 2019 Apr;66(2):369-386. Accessed 11/21/2025.
- National Library of Medicine (U.S.). Phenylketonuria (PKU) Screening (https://medlineplus.gov/lab-tests/phenylketonuria-pku-screening/). Updated 11/8/2022. Accessed 11/21/2025.
- Newborns and Infants. In: David JA, ed. CURRENT Practice Guidelines in Primary Care 2024. McGraw Hill; 2024.
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