Does this test have other names?
PKU screening, Guthrie assay, PKU test
What is this test?
This is a blood test to screen newborns for phenylketonuria (PKU), a condition that
can cause brain damage and severe intellectual disability if it goes untreated. The
problems usually appear in the first year of life, causing infants to appear unusually
sleepy and listless. They may have difficulty feeding and develop a red, itchy rash
similar to eczema. In addition, such babies typically have lighter skin and hair than
family members who don't have the condition.
PKU is a genetic, or inherited, condition. People with PKU don't have the enzyme needed
to process a substance called phenylalanine. This substance is an amino acid that
is a part of proteins found in many foods. Without the enzyme to break it down, phenylalanine
can build up to dangerous levels. People with PKU also lose a substance called phenylacetic
acid in their urine and sweat. If PKU is not treated, they have a distinctive musty
odor. Beginning in infancy and continuing throughout life, people with PKU must follow
a diet that puts strict limits on phenylalanine.
The link between PKU and intellectual disability has been known since the 1930s. In
fact, PKU was the first condition that was screened for in newborns. All U.S. states
screen newborn babies for PKU. This means that almost all cases are now discovered
and treatment started at birth.
Why does my child need this test?
Your child may need this test because finding and beginning treatment of PKU in a
newborn can prevent intellectual disability and other developmental problems in your
child. If your child has a controlled, low-protein diet that carefully limits phenylalanine
in the first weeks of life and beyond, he or she is likely to live a healthy life.
Even though most babies with PKU are diagnosed soon after birth, screening for PKU
should be considered for any child who has an intellectual disability or is developmentally
delayed. Some babies adopted from other countries may also need to be screened for
PKU and other inherited illnesses in the first year of life.
What other tests might my child have along with this test?
Newborns are also tested for other metabolic birth defects before they leave the hospital.
What do my child's test results mean?
Test results may vary depending on your child's age, gender, health history, the
method used for the test, and other things. Your child's test results may not mean
they have a problem. Ask the healthcare provider what the test results mean for your
The test screens for blood levels of phenylalanine. Normal levels of phenylalanine
in the blood are less than 2 milligrams per deciliter (mg/dL). More than 4 mg/dL of
phenylalanine in the blood is considered high and may mean your child has PKU. The
test will be first done after your baby is 24 hours old, then may be repeated when
your baby is 7 to 14 days old.
How is this test done?
Babies are usually screened for PKU with a heel-prick test. This is done by drawing
a few drops of blood from the infant's heel.
A urine test is an alternative to the heel prick. The healthcare provider will collect
a sample of your baby's urine.
Does this test pose any risks?
Having a heel-prick test carries some risks. These include bleeding, infection, or
bruising. When the needle pricks your baby's heel, he or she may feel a slight sting
or pain. Afterward, the site may be sore.
What might affect my child's test results?
Medicines like aspirin or antibiotics may affect the results of the urine test for
PKU. The blood test may give a false-positive or false-negative result in certain
Your baby is premature. This could lead to a false-positive result because certain
liver enzymes have not fully developed.
Your baby has feeding problems such as vomiting. This could give a false-negative
How do I get my child ready for this test?
The test should not be done before 24 hours after birth. If you are breastfeeding,
be sure your healthcare provider knows about all medicines, herbs, vitamins, and
supplements you are taking. This includes medicines that don't need a prescription
and any illicit drugs you may use.