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Phenylketonuria (PKU) Test

Test Overview

A phenylketonuria (PKU) test is done to check whether a newborn baby has the enzyme needed to use phenylalanine in their 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 Canada and the United States have a screening test for PKU to find out if the baby has an increased risk of the disease. To have the disease, you must inherit the gene from each parent.

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. In some areas, 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 lancet. Several drops of blood are collected inside circles on a special piece of paper.

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

PKU screening testfootnote 1

Normal:


Less than 265 micromoles per litre (mcmol/L) or 3 milligrams per decilitre (mg/dL)

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 clear and your baby may need to be tested again include:

  • Your baby was born early (premature). A baby who weighs less than 2.3 kg (5 lb) may have high levels of phenylalanine but not have phenylketonuria (PKU).
  • The PKU screening test was done before your baby was 24 hours old.

What To Think About

  • If your baby has PKU, they will need regular blood tests to check amino acids. 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

  1. 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

Adaptation Date: 8/17/2021

Adapted By: Alberta Health Services

Adaptation Reviewed By: Alberta Health Services

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