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.

• Infant test or procedure preparation (birth to 1 year)

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.

• 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