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. PKU occurs more in whites and Native Americans and is less common in blacks, Hispanics, and Asians. 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.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.
Your baby should be drinking breast milk or formula for 24 hours before the blood sample is taken. PKU test results are more likely to be correct if the blood sample is taken after the baby has been drinking milk or formula for at least 48 hours.
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.
Babies with bleeding problems may bleed more than usual. Sometimes bleeding problems are found when blood is being collected for the PKU test.
If the heel stick test shows high phenylalanine levels, a blood sample is taken from your baby's vein to confirm whether he or she has PKU.
Reasons your baby may not be able to have the test or why the results may not be helpful include:
- U.S. Preventive Services Task Force (2008). Screening for phenylketonuria. Available online:
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
- March of Dimes (2007). PKU (Phenylketonuria). Available online:
- National Institutes of Health (2000). Phenylketonuria (PKU): Screening and management. NIH Consensus Statement, 17(3): 1–33.
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
|Primary Medical Reviewer||John Pope, MD - Pediatrics|
|Specialist Medical Reviewer||Chuck Norlin, MD - Pediatrics|
|Last Revised||September 8, 2011|
Last Revised: September 8, 2011
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