A phenylketonuria (PKU) test is done to check whether a newborn baby has the enzyme needed to use phenylalanine in his or her 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 mental retardation.

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 are tested for PKU right after birth. Each year, about 250 U.S. children get PKU.¹ PKU occurs more in whites and Native Americans and is less common in blacks, Hispanics, and Asians. To have the disease, you must inherit the gene from each parent.

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. A follow-up test is usually done at age 7 to 10 days. A urine PKU test is done on a baby who did not have a blood test and who is older than 6 weeks.

Why It Is Done

A phenylketonuria (PKU) test is done to see whether a newborn baby has the enzyme to use phenylalanine in his or her body.

It is important to test your baby for PKU soon after birth to prevent brain damage. Phenylalanine blood levels in a baby with PKU start to rise within 24 hours of drinking breast milk or formula. A PKU test done 2 to 3 days after birth will show whether your baby has PKU. State laws say that newborn PKU tests must be done within a few days after birth.

How To Prepare

Your baby should be drinking breast milk or formula for 24 hours before the blood sample is taken. PKU test results are more likely to be correct if the blood sample is taken after the baby has been drinking milk or formula for at least 48 hours.

If your baby is older than 6 weeks, he or she will have a PKU urine test. You do not need to do anything before your baby has this test.

How It Is Done

Your baby's heel is cleaned with alcohol and then the heel is poked with a small needle. Several drops of blood are collected inside circles on a special piece of paper. When enough blood has been collected, a small bandage is put on the site.

For a urine test, a drop of special fluid or a special test strip is put on a diaper with fresh urine.