What Is a PKU Test?

Medically Reviewed by Amita Shroff, MD on November 30, 2022
3 min read

Your newborn seems perfect. They've got 10 fingers and toes and  a hearty appetite. But even healthy-looking babies can have problems you can’t see. A simple blood test looks for rare conditions, including phenylketonuria (PKU), which can harm your baby’s growing brain.

PKU is a rare genetic condition that affects metabolism -- the way your body turns food into energy. Babies with PKU can’t make an enzyme needed to break down phenylalanine (Phe) – an amino acid found in protein. Amino acids regulate almost all of the metabolic processes in the human body. If Phe builds up in the blood, it causes damage.

Every baby in the United States is tested for PKU as part of newborn screenings that check for many other conditions, too.

There’s no cure. But with treatment, your baby can grow into a healthy adult.

A health care worker at the hospital will take a blood sample no earlier than 24 hours after your baby is born. If you gave birth outside a hospital, bring your infant to a doctor in a day or two to get all the newborn screenings.

Usually, the blood is taken through a prick in your newborn’s heel. It’s put on a special paper and sent to a lab. Your baby’s doctor will share the results with you.

The test measures the amount of Phe in your baby’s blood.

A normal level is less than 2 milligrams per deciliter (mg/dL). More than 4 mg/dL is considered high. Even if your baby’s results aren’t in that range, it doesn’t necessarily mean they havePKU. It means they need more tests to find out for sure.

If your baby is premature, they might get what’s called a “false positive” because the enzyme to break down Phe hasn’t developed fully yet.

They could get a “false negative” if they aren’t eating (either breastfed or bottle-fed), are throwing up, or if the test was done too soon they were born.

If PKU isn’t diagnosed and treated right away, it can cause lasting problems, including:

If your child has PKU, they need to be on a special, lifelong diet that’s low in Phe. You should start them on it as soon as possible, ideally within a week or 10 days of birth.

Your baby will be put on a Phe-free infant formula. They may be able to have some breast milk, too.

Some kids need a stricter diet than others. A dietitian with expertise in PKU can make an eating plan specific to your child so they grow and thrive.

Your child probably will need to eat carefully measured portions of fruits, vegetables, and low-protein cereals, bread, and pasta.

Foods with higher levels of Phe generally are off-limits. These include:

  • Dairy
  • Eggs
  • Meat, poultry, and fish
  • Nuts
  • Beans
  • Foods or drinks with the artificial sweetener aspartame

Since your child’s diet is limited, they’ll need to drink a special formula that contains protein (minus the Phe) and other nutrients they need.

Regular blood tests and doctor visits help to show whether the diet is working.

Some people take a medicine that helps process Phe such as sapopterin (Kuvan). It’s more likely to work in those with mild or special forms of PKU. Your child will continue their special meal plan, but they may have a bit more freedom while taking the medication.

The control of phenylketonuria levels is a lifelong process. Adults with uncontrolled high concentrations of PKU can be treated with the medication pegvaliase-pqpz (Palynziq) to help reduce levels.