Serum phenylalanine screening

Alternative names
Phenylalanine; Phenylalanine - serum

Serum phenylalanine screening is a test for the disease phenylketonuria (PKU). The test detects abnormally high levels of phenylalanine.

How the test is performed

An area of the infant’s skin, usually the heel, is cleansed with antiseptic and punctured with a sharp needle or a lancet. Three drops of blood are placed in three separate test circles on a piece of paper. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

The test paper is taken to the laboratory where it is mixed with a type of bacteria that needs phenylalanine to grow, and another substance that inhibits phenylalanine from reacting with anything else.

How to prepare for the test
For help preparing your baby for the test, see the following topic:

How the test will feel
When the needle is inserted to draw blood, some infants feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed

This test is performed on infants to screen for phenylketonuria (PKU). This disease is caused by a deficiency of the enzyme needed to breakdown the amino acid phenylalanine.

If PKU is not detected early, an affected infant will become mentally retarded as the amino acid level builds up. However, if it is detected early, the severe side effects of PKU may be avoided through diet modification. Therefore, even though the condition is relatively rare, most newborns are screened.

Normal Values
A normal test results in no bacteria growth. This indicates that phenylalanine levels are normal.

What abnormal results mean

Bacteria growth indicates that phenylalanine levels in the infant’s blood are too high. PKU is a possibility and further testing will be performed.

What the risks are

The risks of having blood drawn are slight, but include:

  • Excessive bleeding  
  • Fainting or feeling lightheaded  
  • Hematoma (blood accumulating under the skin)  
  • Infection (a slight risk any time the skin is broken)  
  • Multiple punctures to locate veins


Johns Hopkins patient information

Last revised: December 5, 2012
by David A. Scott, M.D.

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