Phenylketonuria (PKU) Test
How It Feels
Your baby may feel a sting or a pinch
with a heel stick.
Usually, there are no problems from a heel
stick. A small bruise may develop.
phenylketonuria (PKU) test is done to check whether a
new baby has the
enzyme needed to use phenylalanine in his or her body.
If the heel stick screening test shows high phenylalanine levels,
a blood sample is taken from your baby's vein to confirm whether he or she has
What Affects the Test
Reasons the results may not be helpful include:
- Your baby was born early (premature). A baby
who weighs less than
5 lb (2.3 kg) may have high
levels of phenylalanine but not have phenylketonuria (PKU).
baby has been drinking milk for less than 24 hours. Best results occur after
your baby has been breast-feeding or drinking formula for 2 full
- Your baby is vomiting or refusing to eat. If the PKU test is
done before your baby has eaten for 2 days, the results may not be
- Your baby is getting
What To Think About
- When the PKU test is done within 24 hours
of birth, there is a small chance that the test result will not be accurate (false-negative or false-positive). Your baby may need to be tested
again. There is less chance of a false result if the test is
done between 24 and 72 hours after birth.
- If your baby has PKU, he or she will need regular blood tests to check phenylalanine levels. These tests may occur as often as once a week in your baby's first year and then once or twice a month throughout childhood.
- Blood tests for
phenylalanine may be done if you have PKU and plan to
become pregnant. If you eat too much protein, you will have high levels of
phenylalanine in your blood. If you become pregnant, the high levels of
phenylalanine could cause your baby (fetus) to have
intellectual disability, even if your baby does not have PKU.
- If your baby has PKU, a special low-protein
diet is needed to prevent
intellectual disability. Your baby will drink milk
substitutes that do not contain phenylalanine. People with PKU need to stay on a low-protein diet for
life to prevent problems.