WellSpan Home

Health Library

Phenylketonuria (PKU)

Does this test have other names?

PKU screening, Guthrie assay, PKU test

What is this test?

This is a blood test to screen newborns for phenylketonuria (PKU), a condition that can cause brain damage and severe intellectual disability if it goes untreated. The problems usually appear in the first year of life, causing infants to appear unusually sleepy and listless. They may have difficulty feeding and develop a red, itchy rash similar to eczema. In addition, such babies typically have lighter skin and hair than family members who don't have the condition.

PKU is a genetic, or inherited, condition. People with PKU don't have the enzyme needed to process a substance called phenylalanine. This substance is a part of protein and is found in many foods. Without the enzyme to break it down, phenylalanine can build up to dangerous levels. People with PKU also lose a substance called phenylacetic acid in their urine and sweat. If PKU is not treated, they have a distinctive musty odor. Beginning in infancy and continuing throughout life, people with PKU must follow a diet that puts strict limits on phenylalanine.

The link between PKU and intellectual disability has been known since the 1930s. In fact, PKU was the first condition that was screened for in newborns. All U.S. states screen newborn babies for PKU. This means that almost all cases are now discovered and successfully treated at birth. 

Why does my child need this test?

Finding and treating PKU in a newborn can prevent intellectual disability and other developmental problems in your child. If your child has a controlled, low-protein diet that carefully limits phenylalanine in the first weeks of life and beyond, he or she is likely to live a healthy life.

Even though most babies with PKU are diagnosed soon after birth, screening for PKU should be considered for any child who has an intellectual disability or is developmentally delayed. Some babies adopted from other countries may also need to be screened for PKU and other inherited illnesses in the first year of life. 

What other tests might I have along with this test?

Newborns are also tested for other metabolic birth defects before they leave the hospital. 

What do your child's test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.

The test screens for blood levels of phenylalanine. Normal levels of phenylalanine in the blood are less than 2 milligrams per deciliter (mg/dL). More than 4 mg/dL of phenylalanine in the blood is considered high and may mean your child has PKU. The test will probably be repeated when your baby is 7 to 14 days old. 

How is this test done?

Babies are usually screened for PKU with a heel-prick test. This requires drawing a few drops of blood from the infant's heel.

A urine test is an alternative to the heel prick. The healthcare provider will collect a sample of your baby's urine.

Does this test pose any risks?

A heel-prick test carries risks that include bleeding, infection, or bruising. When the needle pricks your baby's heel, he or she may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore. 

What might affect my child's test results?

Medicines like aspirin or antibiotics may affect the results of the urine test for PKU. The blood test may give a false-positive or false-negative result in certain cases:

  • Your baby is premature. This could lead to a false-positive result because certain liver enzymes have not fully developed.

  • Your baby has feeding problems such as vomiting. This could give a false-negative result.

  • The test is done 3 to 4 days before the baby starts a milk diet. This could give a false-negative result. Some experts believe that for the most accurate results, the test should be done only several days after a milk diet or breastfeeding begins. 

If you are breastfeeding, be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use. 

How do I get my child ready for this test?

The test should not be done before 24 hours after birth.  

Phenylketonuria (PKU) - WellSpan Health

Author: Iliades, Chris, MD
Online Medical Reviewer: Freeborn, Donna, PhD, CNM, FNP
Online Medical Reviewer: Snyder, Mandy, APRN
Last Review Date: 2015-08-29T00:00:00
Last Modified Date: 2015-10-09T00:00:00
Published Date: 2015-10-09T00:00:00
Last Review Date: 2012-04-25T00:00:00
© 2016 WellSpan Health. All Rights Reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.

I would like to:

Are you sure you would like to cancel?

All information will be lost.

Yes No ×

About the provider search

This search will provide you with WellSpan Medical Group and Northern Lancaster County (Ephrata) Medical Group primary care physicians and specialists. If we don’t have a WellSpan Medical Group physician to meet your criteria, the search will expand to include community physicians who partner with WellSpan Medical Group physicians through the WellSpan Provider Network or provide care to patients on the Medical Staffs of WellSpan’s Hospitals.


Schedule Your Next Appointment Online with MyWellSpan

Use your MyWellSpan patient portal any time to view available appointments, and pick the date and time that best suits your schedule.

Go to MyWellSpan

New to this practice?

If you don't have a WellSpan primary care provider and would like to schedule a new patient appointment with a provider who is accepting patients, just log into your MyWellSpan account, and go to the Appointment Center section. As you progress through the scheduling process, you will be able to see the offices that are accepting new patients in relation to your zip code. If you are not enrolled in MyWellSpan, go to https://my.wellspan.org, call 1-866-638-1842 or speak with a member of the staff at a participating facility to sign up. New patient scheduling not available at all practices/programs.

Already a patient at this practice?

If you already have a relationship with a WellSpan practice, simply log into your account, and go to the Appointment Center section. As you progress through the scheduling process, you will be able to schedule an appointment with any provider or practice that already counts you as a patient. Online scheduling varies by practice/program.