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Phenylketonuria (PKU) Screening
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

Phenylketonuria (PKU) Screening

PKU screening is a mandatory newborn test that measures phenylalanine levels to prevent permanent neurological damage through early dietary intervention.

What is a PKU screening test?

A PKU screening test measures the amount of a protein called phenylalanine (Phe) in a newborn's blood. Phe is found in protein-rich foods and the artificial sweetener aspartame. Phenylketonuria (PKU) is a rare genetic disorder where the body cannot properly break down Phe. If left untreated, Phe builds up in the blood and can cause permanent damage to the nervous system and brain, leading to seizures and learning disabilities.

Why does my baby need this test?

All newborns in the United States are required to be tested for PKU, usually between 24 and 72 hours after birth. Early detection allows for a special low-protein diet that can prevent serious health problems. Older infants or children may be tested if they show symptoms such as:

  • Seizures
  • Skin rashes (e.g., eczema)
  • Delayed development
  • Musty odor in breath, skin, or urine
  • Unusually small head (microcephaly)

What happens during the test?

A health care provider cleans the baby's heel with alcohol and uses a small needle to collect a few drops of blood. The test is most accurate after the baby has consumed some protein from breast milk or formula, which is why it is performed a day or two after birth.

What do the results mean?

  • Normal results: It is unlikely your baby has PKU. If the test was performed before 24 hours of age, a follow-up test at 1 to 2 weeks may be required.
  • Not normal results: High levels of Phe were found. Your provider will order follow-up tests to confirm the diagnosis. If PKU is confirmed, the baby will be placed on a special diet or formula that does not contain Phe.

Prenatal Screening

If parents are known carriers of the PKU gene, prenatal screening can be performed using amniocentesis (sampling amniotic fluid) or chorionic villus sampling (sampling placental tissue). Consult with a genetic counselor to determine if this is appropriate for your situation.

Frequently Asked Questions

positive screening result indicates elevated phenylalanine levels, but it is not a definitive diagnosis. It serves as a flag for further testing. Your provider will order a confirmatory plasma amino acid test to measure exact levels of phenylalanine and other amino acids. Only after this confirmatory test can a diagnosis of PKU be made and a specific treatment plan be initiated. How strict is the low-phenylalanine diet for a child with PKU, and can they ever eat normal protein?::The diet is very strict and requires careful monitoring of all food sources, including breast milk, formula, and solid foods. Children with PKU must consume a special medical formula that provides essential amino acids without phenylalanine, along with carefully measured amounts of natural protein. As the child grows, the amount of natural protein allowed may increase, but they will likely need to follow a modified diet for life to prevent neurological damage. What are the long-term consequences if PKU is not treated or if the diet is not followed consistently?::Untreated or poorly managed PKU leads to the accumulation of phenylalanine in the brain, which causes irreversible intellectual disability, seizures, and behavioral problems. It can also result in physical symptoms such as a musty body odor, eczema, and lighter hair and eye color. Consistent adherence to the diet and regular blood monitoring are essential to prevent these permanent neurological and developmental issues. Do I need to test my other children or family members for PKU if one child is diagnosed?::Yes, siblings should be tested because PKU is an autosomal recessive condition, meaning there is a 25% chance for each sibling to be affected if both parents are carriers. Parents should also be tested to confirm carrier status. Genetic counseling is recommended to discuss the inheritance pattern and the risks for future pregnancies, as well as to identify any other family members who may be carriers.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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