Phenylketonuria (PKU) Test

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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 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 Canada and the United States are tested for PKU right after birth. Each year, about 250 U.S. children get PKU.1 PKU occurs more in whites and First Nations and is less common in blacks, Hispanics, and Asians. 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. A follow-up test is usually done at age 7 to 10 days.

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 will 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.


Usually, there are no problems from a heel stick. A small bruise may develop.

Babies with bleeding problems may bleed more with usual. Sometimes bleeding problems are found when blood is being collected for the PKU test.


A phenylketonuria (PKU) test is done to check whether a new baby has the enzyme needed to use phenylalanine in his or her body.


Phenylketonuria (PKU) blood test

Less than 2 milligrams per decilitre (mg/dL) or less than 121 micromoles per litre (mcmol/L) (SI units)

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 2.3 kg (5 lb) 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

  • If your baby goes home within 24 hours of birth, the PKU test result may not be correct. Your baby may need to be tested again if the PKU results are not normal. When the test is done within 24 hours of birth, there is a small chance of having a PKU test be normal when your baby does have PKU. There is a very small 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.
  • Testing 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 counselling. 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 (especially women during their child-bearing years) need to stay on a low-protein diet for life to prevent problems.

Related Information



  1. National Institutes of Health (2006). Fact sheet: Mental retardation. Available online:

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • March of Dimes (2007). PKU (Phenylketonuria). Available online:
  • 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.


By Healthwise Staff
Primary Medical Reviewer Susan C. Kim, MD - Pediatrics
Primary Medical Reviewer Anne C. Poinier, MD - Internal Medicine
Specialist Medical Reviewer Thomas Emmett Francoeur, MD, MDCM, CSPQ, FRCPC - Pediatrics
Last Revised June 8, 2010

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