Serum Phenylalanine Screening

Written by Gretchen Holm | Published on June 1, 2012
Medically Reviewed by Jennifer Wider, MD

What Is a Serum Phenylalanine Screening?

A baby born with phenylketonuria (PKU) lacks a specific enzyme called phenylalanine hydroxylase. This enzyme is what allows the body to break down phenylalanine, an essential nutrient that your body requires, but can’t create itself. Without this enzyme, phenylalanine builds up in your baby’s body. This can lead to brain damage and mental retardation.

Fortunately, this condition is rare. It is an inherited condition in which both parents must pass on a defective gene for a baby to have this condition. This is called an autosomal recessive trait.

By testing your newborn’s blood for excess phenylalanine, doctors can determine whether he or she has PKU. The serum phenylalanine screening is a test that should be performed on all infants within the first two to three days of life.

Why Doctors Perform a Serum Phenylalanine Screening

Doctors perform a serum phenylalanine test as part of a routine screening for newborns. This means it’s perfectly normal (and expected) for your doctor to order this test. It doesn’t mean that your doctor suspects any problems with your baby. In fact, newborns with PKU don’t show any physical signs of the disease.

Because PKU is treatable and symptoms do not appear until the affected child is a few months old, routine screening is performed at birth in the United States and in many other countries.

Serum Phenylalanine Screening

This test is a simple blood test. The doctor will clean a patch of skin, usually on your baby’s heel. He or she will then prick the skin with a needle or lancet (a small surgical knife). Next, your baby’s doctor will collect several drops of blood. This blood will then be tested.

Risks of the Serum Phenylalanine Screening

The slight risks associated with this test are present any time a blood sample is taken. These include:

  • infection
  • excessive bleeding
  • fainting
  • hematoma
  • inflammation of the vein

Your baby may feel mild to moderate pain when the skin is punctured. He or she might also feel throbbing in the puncture site for a few minutes after the procedure. The site might continue to bleed slightly for a few more minutes, and your baby may develop a small bruise in the area.

Serum Phenylalanine Screening Results

Phenylalanine screening results are measured in milligrams per deciliter (mg/dL). A normal result is 4 mg/dL or lower.

If your baby’s serum phenylalanine screening result is greater than 4 mg/dL, further testing with a slightly larger sample of blood is required.

Dealing with PKU

If your baby has PKU, you can treat the disease. PKU is the inability to break down phenylalanine. Therefore, your baby will need to follow a diet that is extremely low in phenylalanine. This involves limiting or eliminating high-protein foods from your baby’s diet.

Specific formulas that are free of phenylalanine are available. There is also a supplement known as a neutral amino acid, which comes as both a powder and a tablet. It blocks the absorption of phenylalanine. Your doctor will discuss what is most appropriate for your child.

This diet should continue throughout your baby’s life, even when he or she grows up. Your doctor will be able to refer you to a dietician who will help you understand exactly which foods to avoid or to give your baby only in moderation.

Some examples of foods that are high in phenylalanine include:

  • dairy products
  • nuts
  • peas and beans (including soybeans)
  • meats
  • chocolate
  • beer
  • foods containing aspartame (NutraSweet, Equal), such as diet sodas and foods with artificial sweeteners

Your doctor may instruct you to give your baby supplements to make up for the nutrients missing from a phenylalanine-low diet. Possible supplements include fish oil, iron, and carnitine, an amino acid that has similar properties to phenylalanine.

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