Understanding the PKU Test

November 12, 2012

Understanding the PKU Test

Even though a PKU child can seem perfectly healthy for the first few months of life, the lack of a PKU test can lead to irreversible brain damage.

Phenylketonuria (PKU) is a genetic disorder that occurs when each parent passes on an abnormal PAH gene to their child. People with only one abnormal gene do not have any noticeable health problems but those with two abnormal genes will develop the disease. The PAH gene is responsible for regulating the formation of the enzyme needed to adequately process the protein phenylalanine. Because many foods are very high in this particular protein, individuals who cannot break it down have elevated levels of the protein in their body. With immediate and long term care often necessary to prevent negative side effects, the PKU test was the United States’ first screening test to be used on all newborns.

How the PKU Test Works

All infants are screened within the first few days of life for Phenylketonuria. On average, 1 in 25,000 babies is afflicted with this condition. The PKU test is a simple procedure which will allow doctors to see if there is an abnormal rise in phenylalanine blood levels. A small amount of blood is taken from the baby through a heel stick. The great thing about this test is that the blood can also be used to test for a number of other conditions that may be of interest to the physician. Abnormal results will lead to further testing in order to pinpoint what is causing the rise in the protein. Because cases can be missed with PKU tests performed in the first 24 hours of life, it is recommended that parents have their infants retested after a few weeks.

Symptoms of PKU

As the phenylalanine levels rise in the blood, the protein can have irreversible effects on the body. The signs and symptoms of the condition can vary and often appear when levels have been too high for way too long. The PKU test is used to catch the elevation before symptoms set in. While many infants appear normal for the first few months of life, symptoms which will eventually appear include


  • By 3-6 months of age, infants will stop caring about their surroundings

  • By one year of age, obvious delays in mental development will be noticeable

  • Irritability

  • Behavioral problems

  • Seizures

  • Rashes

  • Musty body odor

  • Abnormally dry skin

  • Tendency to have blonder/lighter hair than family members

In the past, doctors believed it was safe to end the PKU specialized diet after a certain age. The problem with this is that pregnant PKU positive women had excessively high levels of the protein which was unsafe for the fetus. Signs that an infant has been affected by this situation are:

  • Mental retardation

  • Microcephaly (small heads)

  • Low birth-weight

  • Heart defects

A PKU test is important because once these signs and symptoms appear it is often not possible to correct the health problems.

Treating PKU

Once an infant is suspected of having PKU, it is necessary to put him or her on a special diet. This diet should be started as soon as possible in order to prevent mental retardation. Foods and formula very low in phenylalanine should be used so that excessive amounts of the protein do not have a chance to build up. Common safe foods include some grain products, certain veggies, and a variety of fruits. No high protein foods such as fish, eggs, or cheese should be consumed.

A special protein formula should be used throughout childhood so that the body still has access to other essential proteins. All people with a positive PKU test will require blood level monitoring on a regular basis. As a person gets older, the diet restrictions may become more lax but should still be closely regulated. Currently, there is one FDA approved drug for the management of PKU. Individuals on Kuvan should still watch their diet and visit the doctor regularly.

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