Test Overview
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
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 his or her 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
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.
How It Feels
Your baby may feel a sting or a pinch
with a heel stick.
Risks
Usually, there are no problems from a heel
stick. A small bruise may develop.
Results
A
phenylketonuria (PKU) test is done to check whether a
new baby has the
enzyme needed to use phenylalanine in his or her body.
Normal
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
PKU.
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).
- Your
baby has been drinking milk for less than 24 hours. Best results occur after
your baby has been breastfeeding or drinking formula for 2 full
days.
- 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
correct.
- Your baby is getting
antibiotics.
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.
References
Citations
- U.S. Preventive Services Task Force (2008). Screening for phenylketonuria (PKU). Available online: http://www.uspreventiveservicestaskforce.org/uspstf/uspsspku.htm.
- Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
Other Works Consulted
- Committee on Genetics, American Academy of Pediatrics (2008). Maternal phenylketonuria. Pediatrics, 122(2): 445-449.
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
- March of Dimes (2013). Birth defects: PKU in your baby. Available online: http://www.marchofdimes.com/baby/birthdefects_pku.html.
- Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Credits
ByHealthwise Staff
Primary Medical ReviewerJohn Pope, MD - Pediatrics
U.S. Preventive Services Task Force (2008). Screening for phenylketonuria (PKU). Available online: http://www.uspreventiveservicestaskforce.org/uspstf/uspsspku.htm.
Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.