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Phenylketonuria (PKU) Test

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 test 2
Normal:

Less than 3 milligrams per deciliter (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 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.

WebMD Medical Reference from Healthwise

Last Updated: June 26, 2013
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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