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, which means that almost all cases are now discovered and successfully treated at birth. 

Why do I need this test?

Detecting 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 often tested for other metabolic birth defects before they leave the hospital. 

What do my 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 health care 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 doctor 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 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 three to four 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 doctor 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 ready for this test?

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