Serum phenylalanine screening is a blood test to look for signs of the disease phenylketonuria (PKU). The test detects abnormally high levels of an amino acid called phenylalanine.

This test is done to screen infants for phenylketonuria (PKU), a relatively rare condition that occurs when the body lacks a substance needed to breakdown the amino acid phenylalanine.

If PKU is not detected early, increasing phenylalanine levels will cause mental retardation. When discovered early, changes in the diet can help prevent the severe side effects of PKU.

The test is usually included in routine screening tests, which are done before the newborn leaves the hospital. If the child is not born in the hospital, the test should be done in the first 48 to 72 hours of life.

An area of the infant's skin, usually the heel, is cleaned with a germ killer (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 blocks phenylalanine from reacting with anything else.

See also: Newborn screening tests

For help preparing your baby for the test, see infant test or procedure preparation (birth to 1 year).

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. Infants are given a small amount of sugar water, which has been shown in studies to reduce the painful sensation associated with the skin puncture.