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Neonatal Screening: What Is Guthrie Test & What Can It Tell You?

By Bindu Vinodh

Did you know that while most babies look healthy, there are some disorders that may not be externally visible? Early diagnosis and treatment of such disorders can help prevent serious complications in the future that may otherwise cause permanent damage to the baby or may even turn out to be a life-threatening condition. Guthrie Test is a neonatal screening done to check for one such disorder called Phenylketonuria (PKU).

neonatal screening

PKU is a congenital birth abnormality characterized by the presence of high levels of amino acid, Phenylalanine (Phe) in the blood of neonates.

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How Is Guthrie Test Done?

To do the test, a blood sample is taken from the baby's heel within 48 to 72 hours of age and tested. In case high levels of Phe are detected in the blood sample, the baby is placed on a Phe-restricted diet for a few years.

This can offset the neurological disorders that are expected with this condition. As the blood sample is taken from the baby's heel, it is also referred to as the ‘Heel Prick' test.

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What Does The Guthrie Test Tell You?

The Guthrie Test was the first widely used perinatal biochemical test introduced since the early 1960s. Guthrie Test also known as PKU test or the ‘heel-prick test' is a diagnostic tool to test for phenylketonuria in infants.

The test detects phenylalanine in the blood of newborns, helping with early diagnosis of PKU. This helps prevent the development of the mental disabilities in thousands of individuals, who are affected by this condition annually.

Basically, PKU is an inherited metabolic disease, wherein the body does not produce the enzyme, phenylalanine hydroxylase. This enzyme is necessary to break down the amino acid phenylalanine in the blood. Individuals with PKU cannot digest high concentrations of this amino acid, which is present in protein-rich foods.

When this happens, the high levels of phenylalanine build up in blood, which leads to brain damage, mental disabilities and seizures. As per latest evidence, high levels of Phe may impair mental performance of the child later in life, and therefore, a Phe-restricted diet is recommended.

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What To Do If The Guthrie Test Is Positive?

The blood sample for PKU test screening should be obtained within 48 hours after the birth of the baby. The normal level of Phe in a healthy baby's blood is less than 2 mg/dL. A level more than 4 mg/dL is considered high.

Sometimes, the test may show a ‘false positive', if your baby is premature, as the enzyme to break down the Phe would not have developed fully yet. Also, at times, a ‘false negative' may appear on the test if the test is done too soon after the baby was born, or if the baby has been throwing up often.

After the initial newborn screening test, a second test is done for verification and if it shows a positive result, a confirmatory quantitative test such as ‘McCamon-Robins Flurometric Test' is done by the laboratory at a referral metabolic centre.

The baby should be referred to a centre that is capable of providing the necessary medical, nutritional and laboratory services for babies with PKU. Apart from the Phe-restricted diet, the baby may require the assistance of professionals experienced in the management of PKU, and follow-up developmental testing is highly recommended.

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Signs And Symptoms Of PKU

The signs and symptoms may vary from mild to severe depending on phenylalanine levels. If left undiagnosed and untreated, the infants appear normal until a few months old, and then begin to show few of these symptoms:

• Intellectual disability
• Behavioral problems
• Seizures
• Delayed development
• Psychiatric disorders
• Children with untreated PKU may have lighter skin and hair too

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How To Manage PKU In Babies?

If your baby is diagnosed with PKU, the baby needs to be on a special Phe-low diet for the next couple of years or may be life-long, depending on the severity of the condition. The diet should begin as early as possible, ideally, within a week or 10 days after birth. If the baby is formula-fed, he or she would be put on a Phe-free infant formula.

A dietician who is also an expert in PKU can help you in making a specific eating plan so that your child grows to be healthy and active. Your child may have to eat carefully measured portions of low-protein cereals, vegetables, fruits, pasta and bread.

Just to help you in managing better, here's the list of foods with higher levels of Phe: dairy, nuts, eggs, fish, poultry, meat, foods or drinks with artificial sweetener, beans and aspartame.

Due to the restrictions in your child's diet, the child may be prescribed a special protein formula (devoid of Phe) and other nutrients that he or she may need.

Some people are seen to take the help of a medicine to process Phe, such as sapoprotein. It will, however, work only on those with mild or special forms of PKU.

Controlling phenylketonuria levels is a life-long process. All this needs to be followed up with regular blood tests, doctor visits etc., to check if the diet works. Adults diagnosed with PKU are treated with medication to lower the Phe levels.

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