Skip to Content
Home > Health Information > Health Library > Phenylketonuria (PKU) Test
A phenylketonuria (PKU) test is done to check whether a newborn baby has the enzyme needed to use phenylalanine in his or her body. Phenylalanine is an amino acid that is needed for normal growth and development. If a baby's body does not have the enzyme that changes phenylalanine into another amino acid called tyrosine, the phenylalanine level builds up in the baby's blood and can cause brain damage, seizures, and intellectual disability.
The damage caused by PKU can begin weeks after the baby has started drinking breast milk or formula. Babies with PKU need foods low in phenylalanine to prevent severe brain damage. Phenylalanine is found in most foods that have protein, such as milk, cheese, and meats.
It is important to find this disease early. All babies in the United States and Canada are tested for PKU right after birth. To have the disease, you must inherit the gene from each parent. The United States Preventive Services Task Force recommends that all newborns be tested for PKU.footnote 1
The blood sample for PKU is usually taken from your baby's heel (called a heel stick). The test is done in the first few days after birth, as early as 24 hours after birth. The test may be repeated within the first week or two after birth.
A phenylketonuria (PKU) screening test is done to see whether a newborn baby has the enzyme needed to use phenylalanine in his or her body. If this test shows that your baby has a phenylalanine problem, the doctor will do further testing to check whether your baby has PKU.
It's important for your baby to have this screening test soon after birth. If a baby has PKU and treatment starts right away, problems (such as brain damage) are less likely to occur.
You do not need to do anything before your baby has this test.
Your baby's heel is cleaned with alcohol, and then the heel is poked with a small needle. Several drops of blood are collected inside circles on a special piece of paper. When enough blood has been collected, a small bandage is put on the site.
Your baby may feel a sting or a pinch with a heel stick.
Usually, there are no problems from a heel stick. A small bruise may develop.
A phenylketonuria (PKU) test is done to check whether a new baby has the enzyme needed to use phenylalanine in his or her body.
Less than 3 milligrams per deciliter (mg/dL)
If the heel stick screening test shows high phenylalanine levels, a blood sample is taken from your baby's vein to confirm whether he or she has PKU.
Reasons the results may not be helpful include:
U.S. Preventive Services Task Force (2008). Screening for phenylketonuria (PKU). Available online: http://www.uspreventiveservicestaskforce.org/uspstf/uspsspku.htm.
Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
Other Works Consulted
Committee on Genetics, American Academy of Pediatrics (2008). Maternal phenylketonuria. Pediatrics, 122(2): 445–449.
Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
March of Dimes (2013). Birth defects: PKU in your baby. Available online: http://www.marchofdimes.com/baby/birthdefects_pku.html.
Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Current as of:
December 12, 2018
Author: Healthwise StaffMedical Review: John Pope MD - PediatricsKathleen Romito MD - Family Medicine
Current as of:
December 12, 2018
Medical Review:John Pope MD - Pediatrics & Kathleen Romito MD - Family Medicine
To learn more about Healthwise, visit Healthwise.org.
© 1995-2019 Healthwise, Incorporated. Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated.
Our interactive Decision Points guide you through making key health decisions by combining medical information with your personal information.
You'll find Decision Points to help you answer questions about:
Get started learning more about your health!
Our Interactive Tools can help you make smart decisions for a healthier life. You'll find personal calculators and tools for health and fitness, lifestyle checkups, and pregnancy.
Feeling under the weather?
Use our interactive symptom checker to evaluate your symptoms and determine appropriate action or treatment.
See important information from Billings Clinic on Non-discrimination and Interpreter Services.
Billings Clinic is proud to be an equal opportunity/affirmative action (EOE/AA) e-verify employer.