PKU screening, Guthrie assay, PKU 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 often appear in the first year of life, causing infants to appear abnormally sleepy and listless. They may have trouble 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 an amino acid that is a part of proteins found in many foods. Without the enzyme to break it down, phenylalanine can build up to dangerous levels in the body. People with PKU also lose a substance called phenylacetic acid in their urine and sweat. If PKU isn't treated, they have a distinctive musty odor. Starting in infancy and all through their life, people with PKU must follow a diet that puts strict limits on how much phenylalanine they can eat.
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 found and treatment started at birth.
Your child may need this test because finding and starting treatment of 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, they are 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.
Newborns are also tested for other metabolic birth defects often before they leave the hospital.
Test results may vary depending on your child's age, gender, health history, the method used for the test, and other things. Your child's test results may not mean they have a problem. Ask the healthcare provider what the test results mean for your child.
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 be first done after your baby is 24 hours old, then may be repeated when your baby is 7 to 14 days old.
Babies are often screened for PKU with a heel-prick test. This is done by getting 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.
Having a heel-prick test carries some risks. These include bleeding, infection, or bruising. When the needle pricks your baby's heel, they may feel a slight sting or pain. Afterward, the site may be sore.
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.
Medicines such as aspirin or antibiotics may affect the results of the urine test for PKU.
The test should not be done before 24 hours after birth. 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 illegal drugs you may use.