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. 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
United States Preventive Services Task Force
recommends that all newborns be tested for PKU.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. 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 needed 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.
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 may 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.
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.
Babies with bleeding problems
may bleed more than usual. Sometimes bleeding problems are found when blood is
being collected for the PKU test.
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 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 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).
- Your
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
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.
- 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.
References
Citations
- U.S. Preventive Services Task Force (2008). Screening for phenylketonuria (PKU). Available online: http://www.uspreventiveservicestaskforce.org/uspstf/uspsspku.htm.
- Fischbach FT, Dunning MB III, eds. (2009).
Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis:
Saunders.
- March of Dimes (2007). PKU (Phenylketonuria). Available online:
http://www.marchofdimes.com/baby/birthdefects_pku.html.
- National Institutes of Health (2000). Phenylketonuria
(PKU): Screening and management. NIH Consensus Statement, 17(3): 1–33.
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Credits
| By | Healthwise Staff |
|---|
| Primary Medical Reviewer | John Pope, MD - Pediatrics |
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| Specialist Medical Reviewer | Chuck Norlin, MD - Pediatrics |
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| Last Revised | September 8, 2011 |
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U.S. Preventive Services Task Force (2008). Screening for phenylketonuria (PKU). Available online: http://www.uspreventiveservicestaskforce.org/uspstf/uspsspku.htm.
Fischbach FT, Dunning MB III, eds. (2009).
Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
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