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.
Babies with bleeding problems
may bleed more than usual. Sometimes bleeding problems are found when blood is
being collected for the PKU test.
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 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 your baby may not be
able to have the test or why 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.
- Sick babies or babies
born early are usually tested for PKU soon after birth and then again at age 2
to 3 weeks.
- 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.
- A blood test 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. It is
important that everyone with PKU see a specialist for nutritional counseling.
Phenylalanine levels should be below 6 mg/dL at least 3 months before you
become pregnant. Recommended phenylalanine levels during pregnancy are 2 mg/dL
to 6 mg/dL, and you should be tested at least once a week.
- A test
for phenylalanine levels in urine may be done if your baby is now over 6 weeks
of age and did not have a PKU blood test 2 to 3 days after birth. A PKU heel
stick can be done up to 6 weeks of age and has better results than a urine
test. A urine test may be done to check phenylalanine levels during treatment
with low-protein foods.
- 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.