What Is a PKU Test?

Your newborn seems perfect. She’s got 10 fingers and toes and already has 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.

When and How Is the Test Done?

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.

What Do the Results Mean?

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 she has PKU. It means she needs more tests to find out for sure.

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

She could get a “false negative” if she isn’t eating (either breastfed or bottle-fed), is throwing up, or if the test was done too soon she was born.

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Why Is the Test So Important?

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

How Is It Treated?

If your child has PKU, she needs to be on a special, lifelong diet that’s low in Phe. You should start her 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. She 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 she grows and thrives.

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, she’ll need to drink a special formula that contains protein (minus the Phe) and other nutrients she needs.

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 her special meal plan, but she 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 the medication pegvaliase-pqpz (Palynziq) to help reduce levels.

WebMD Medical Reference Reviewed by Louise Chang, MD on August 16, 2018

Sources

SOURCES:

CDC: “Newborn Screening.”

The Nemours Foundation: “Newborn Screening Tests.”

National PKU Alliance: “About PKU.”

The University of Rochester Medical Center: “Phenylketonuria (PKU).”

March of Dimes: “PKU (Phenylketonuria) In Your Baby.”

Academy of Nutrition and Dietetics: “How to Create a PKU-Friendly Diet.”

Eunice Kennedy Shriver National Institute of Child Health and Human Development: “What are common treatments for phenylketonuria (PKU)?”

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