What Is Phenylketonuria (PKU) Test or Newborn Screening? | Nutrition

Phenylketonuria (PKU) Test or Newborn Screening

Overview of phenylketonuria (PKU) test?

Phenylketonuria (PKU) test is a rare genetic disorder that causes an amino acid named phenylalanine to build up in the body. Amino acids are the building blocks of protein. Phenylalanine is originated in all proteins and some artificial sweeteners.

Phenylalanine hydroxylase is an enzyme that your body uses to change phenylalanine to tyrosine, which your body needs to make neurotransmitters like epinephrine, norepinephrine, and dopamine. PKU is caused by a defect in the gene that helps make phenylalanine hydroxylase. When this enzyme is lost, your body cannot break down phenylalanine. This causes a build-up of phenylalanine in your body.

Babies in the United States are screened for PKU shortly after birth. The condition is rare in this country, affecting only about 1 in 10,000 to 15,000 newborns each year. Serious signs and symptoms of PKU are rare in the United States, as early detection allows treatment to begin soon after birth. Early diagnosis and treatment can help relieve PKU symptoms and prevent brain damage.

Symptoms of phenylketonuria

The symptoms of PKU can variety from mild to severe. The most serious form of this disorder is known as classic PKU. A baby with classic PKU may appear normal for the first few months of her life. If the baby is not treated for PKU during this time, she will begin to develop the following symptoms:

  • convulsions
  • shaking or shaking and shaking
  • stunted growth
  • hyperactivity
  • skin conditions such as eczema
  • a musty smell on your breath, skin, or urine

If PKU is not diagnosed at birth and treatment is not started quickly, the disorder can cause:

  • Irreversible brain damage and intellectual disability during the first months of life.
  • behavior problems and seizures in older children
  • A less severe form of PKU is called variant PKU or hyperphenylalaninemia without PKU. This happens when the baby has too much phenylalanine in her body. Babies with this form of the disorder may have only mild symptoms, but they will need to be on a special diet to prevent intellectual disabilities.
  • Once a specific diet and other necessary treatments are started, the symptoms begin to subside. People with PKU who manage their diet properly generally do not show any symptoms.

Types of phenylketonuria test

Classic PKU is an autosomal recessive disorder, caused by mutations in both alleles of the phenylalanine hydroxylase (PAH) gene, which is found on chromosome 12. In the body, phenylalanine hydroxylase changes the amino acid phenylalanine to tyrosine, an additional amino acid.

Causes of phenylketonuria

The PAH gene provides orders for making an enzyme called phenylalanine hydroxylase. This enzyme changes the amino acid phenylalanine into other significant compounds in the body. If genetic changes reduce the activity of phenylalanine hydroxylase, phenylalanine in the diet is not processed efficiently. As a consequence, this amino acid can build up to toxic levels in the blood and other tissues. Because nerve cells in the brain are chiefly sensitive to phenylalanine levels, excessive amounts of this substance can cause brain damage.

Classic PKU, the plainest form of the disorder, occurs when phenylalanine hydroxylase activity is severely abridged or absent. People with untreated classic PKU have phenylalanine levels high enough to cause severe brain damage and other serious health problems. Mutations in the PAH gene that allow the enzyme to retain some action result in milder versions of this condition, such as variant PKU or hyperphenylalaninemia without PKU.

Changes in other genes can influence the severity of PKU, but little is known about these additional genetic factors.

Phenylketonuria (PKU) treatment

There is no cure for phenylketonuria, but treatment can prevent intellectual incapacity and other health problems.1 A person with PKU should be treated at a medical center that specializes in the disorder. (Visit the Resources and Publications unit for ways to locate a center.)

The PKU diet

People with PKU should follow a diet that limits foods with phenylalanine. The diet should be followed carefully and started as soon as possible after birth. In the past, experts believed that it was safe for people to stop dieting as they got older. However, they now recommend that people with PKU follow the diet throughout their lives for better physical and mental health.1,2

It is particularly important that a pregnant woman with PKU strictly follow the low phenylalanine diet throughout the pregnancy to safeguard the healthy development of her baby.

People with PKU should avoid several high-protein foods, including:

  • Milk and cheese
  • Eggs
  • Walnuts
  • Soy
  • Beans
  • Chicken, beef, or pork
  • Fish
  • Peas
  • Beer

People with PKU should also avoid the sweetener aspartame, which is found in some foods, beverages, medications, and vitamins. Aspartame releases phenylalanine when digested, thus increasing the level of phenylalanine in a person’s blood.

Often, people with PKU must also limit their intake of foods that are low in protein, such as certain fruits and vegetables. However, a diet for PKU may include low-protein noodles and other specialty items.

The quantity of phenylalanine that is safe to consume differs for each person. Therefore, a person with PKU must work with a healthcare professional to develop an individualized diet. The goal is to invest only the amount of phenylalanine necessary for healthy growth and body processes, but not more. Frequent blood tests and physician visits are necessary to help conclude how well the diet is working. The diet may relax a bit as the child grows, but the current recommendation is adherence to the diet throughout life.2,4 Following the diet is especially important during pregnancy.

However, the PKU diet can be very challenging. Receiving support from friends and family or a support group can help. Sticking to the diet ensures better functioning and overall better health.

A formula for PKU

People on the PKU diet will not get enough essential nutrients from food. Therefore, they must drink a special formula.

A newborn diagnosed with PKU should receive special infant formula. The formula can be mixed with a small aggregate of breast milk or regular infant formulation to make sure the child gets enough phenylalanine for normal development, but not enough to cause harm.

Older children and adults are given a different formula to meet their nutritional needs. This formula must be consumed every day during a person’s life.

In addition to the formula, health professionals may recommend other supplements. For example, fish oil may be optional to help with fine motor organization and other aspects of development.

Medicines for phenylketonuria

The US Food and Drug Administration (FDA) has approved the drug sapropterin dihydrochloride (Kuvan®) for the treatment of PKU. Kuvan® is a form of BH4, which is a material in the body that helps break down phenylalanine. However, having too little BH4 is just one of the reasons a person cannot break down phenylalanine. Therefore, Kuvan® only helps some people reduce phenylalanine in the blood. Even if the medicine helps, it will not lower the phenylalanine to the desired amount and should be used in conjunction with the PKU diet.

When the FDA approved Kuvan®, the agency suggested that research on the drug continue to determine its long-term safety and efficacy.

Other treatments for PKU

Researchers supported by the NICHD and other scientists are exploring additional treatments for PKU. These treatments include great supplements of neutral amino acids, which can help prevent phenylalanine from entering the brain, and enzyme replacement therapy, which uses an enzyme-like substance that generally breaks down phenylalanine. Investigators are also investigating the possibility of using gene therapy, which involves injecting new genes to break down phenylalanine. That would result in the breakdown of phenylalanine and decreased levels of phenylalanine in the blood.

Possible complications

Severe mental disability occurs if the disorder is not treated. ADHD (attention deficit hyperactivity disorder) seems to be a common problem in those who do not follow a diet very low in phenylalanine.


An enzyme assay or genetic test can determine if parents carry the PKU gene. Chorionic villus sampling or amniocentesis may be done during pregnancy to test the fetus for PKU.

It is very important for women with PKU to strictly follow a low-phenylalanine diet both before becoming pregnant and throughout pregnancy. The accumulation of phenylalanine will harm the developing baby, even if the child has not inherited the entire disease.

Alternative names

PKU; Neonatal phenylketonuria

Phenylketonuria (PKU) diagnosis

Newborn blood tests identify almost all cases of phenylketonuria. All 50 states in the United States require newborns to be partitioned for phenylketonuria. Many other countries also routinely screen babies for phenylketonuria.

If you have phenylketonuria or a family history of it, your doctor may recommend screening tests before pregnancy or delivery. phenylketonuria carriers can be identified by a blood test.

Testing your baby after birth

The Phenylketonuria (PKU) test is done a day or two after your baby is born. The test is done after your baby is 24 hours and after your baby has eaten some protein in the diet to ensure accurate results.

A nurse or lab technician collects a few drops of blood from your baby’s heel or the crease of your baby’s arm. A research laboratory tests the blood sample for certain metabolic disorders, including phenylketonuria.

If you do not deliver your baby in a hospital or are discharged soon after birth, you may need to schedule a newborn screening test with your pediatrician or family doctor.

If this test indicates that your baby may have phenylketonuria:

  • Your baby may have extra tests to confirm the diagnosis, including more blood tests and urinalysis.
  • You and your baby can undergo genetic testing to identify genetic mutations.
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