Skip to content
My WebMD Sign In, Sign Up

Health & Baby

Font Size

Phenylketonuria (PKU) Test

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.

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.
  • 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.
  • 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.

Citations

  1. U.S. Preventive Services Task Force (2008). Screening for phenylketonuria (PKU). Available online: http://www.uspreventiveservicestaskforce.org/uspstf/uspsspku.htm.

  2. 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.

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.

Today on WebMD

mother on phone holding baby
When you should call 911.
Mother with baby
Unexpected ways your life will change.
 
baby acne
What’s normal – and what’s not.
baby asleep on moms shoulder
Help your baby get the sleep he needs.
 

mother holding baby at night
ARTICLE
mother with sick child
QUIZ
 
baby with pacifier
VIDEO
Track Your Babys Vaccines
TOOL
 
Baby Napping 10 Dos And Donts
Slideshow
Woman holding feet up to camera
Article
 
Father kissing newborn baby
Article
baby gear slideshow
Slideshow