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Phenylketonuria (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

Phenylketonuria (PKU) blood test2
Normal:

Less than 2 milligrams per deciliter (mg/dL) or less than 121 micromoles per liter (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 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.
  • 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.

WebMD Medical Reference from Healthwise

Last Updated: June 19, 2012
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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