Phenylketonuria (PKU) Test

Test Overview

A phenylketonuria (PKU) test is done to check whether a newborn baby has the enzyme needed to use phenylalanine in their 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. 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.^{footnote 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.

Why It Is Done

A phenylketonuria (PKU) screening test is done to see whether a newborn baby has the enzyme needed to use phenylalanine in their 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.

How To Prepare

You do not need to do anything before your baby has this test.

How It Is Done

A heel stick is used to get a blood sample from a baby. The baby's heel is poked, and several drops of blood are collected. Your baby may have a tiny bruise where the heel was poked.

How It Feels

A brief pain, like a sting or a pinch, is usually felt when the lancet punctures the skin. Your baby may feel a little discomfort with the skin puncture.

Risks

There is very little risk of a problem from a heel stick. Your baby may get a small bruise at the puncture site.

Results

If the heel stick screening test shows high phenylalanine levels, a blood sample is taken from your baby's vein. This is to confirm if your child has PKU.