Phenylketonuria (PKU) Screening
Reviewed
Also called: PKU newborn screening, PKU test
Screening test for phenylketonuria (PKU) in newborns, measuring phenylalanine (Phe) levels in blood. A screen-negative result means lower likelihood of PKU; a screen-positive result means higher likelihood and requires prompt confirmatory testing. Early detection and dietary management can prevent lifelong brain damage.
Specimen: Blood (heel stick)
Why this test may be ordered
- Routine newborn screening required for all newborns in the United States, usually performed 24-72 hours after birth.
- Older infants or children who were not screened at birth or have symptoms of PKU (e.g., seizures, developmental delays, musty odor) may be tested.
Preparation and collection
No special preparation needed. For accurate results, the baby should have had some protein from breast milk or formula before the test.
A health care provider cleans your baby's heel with alcohol, then sticks the heel with a small needle to collect a few drops of blood. The heel is then bandaged. For accurate results, the baby should have had some protein from breast milk or formula before the test.
There is very little risk 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.
Understanding results
The PKU screening test measures the amount of phenylalanine (Phe) in your baby's blood. High levels may indicate phenylketonuria (PKU), a rare genetic disorder that can cause brain damage if untreated. The test is usually done 24-72 hours after birth. Results are reported as a likelihood of PKU and require follow-up with a healthcare provider for interpretation.
- Screen-negative (lower-likelihood of PKU): The amount of phenylalanine in the baby's blood was within normal range. The baby is unlikely to have PKU. Limitation: Even with a screen-negative result, if both parents are known carriers of a PKU gene, genetic testing is recommended to definitively rule out PKU. Next step: Routine follow-up as per newborn care. If the test was done before 24 hours after birth, a repeat test at 1-2 weeks of age may be recommended.
- Screen-positive (higher-likelihood of PKU): High levels of phenylalanine were found in the baby's blood. The baby most likely has PKU. Limitation: This result indicates a high likelihood of PKU but is not a confirmed diagnosis. Additional testing is needed to confirm. Next step: Prompt follow-up with the healthcare provider. Additional blood and/or urine tests will be arranged to confirm or rule out PKU. If PKU is confirmed, a special low-protein diet and formula without phenylalanine can be started to prevent health problems. Discuss breastfeeding with the provider.
- The test is a screening, not a definitive diagnosis. Further testing is needed to confirm or rule out PKU.
- If performed before 24 hours after birth, a repeat test at 1-2 weeks of age may be necessary.
- There is a small risk of false positives or negatives; results should be interpreted by a healthcare professional.
What to do next
Discuss results with your baby's healthcare provider. If screen-positive, prompt follow-up for confirmatory testing and potential dietary management. If screen-negative but both parents are known carriers, genetic testing may be recommended.
- What does my baby's test result mean?
- Is further testing needed?
- What are the next steps for my baby's health?
- How can I ensure my baby's safety if the result is screen-positive?
Evidence source
U.S. National Library of Medicine: Phenylketonuria (PKU) Screening