In the United States, all babies are required to have a blood test shortly after birth to screen for several serious diseases. The purpose of screening is to protect children with these diseases before any symptoms arrive. In order for a test to be included in the newborn screen, the associated disease should be relatively common in the screened population, the consequences of the untreated disease should be substantial, and the test should be accurate. In addition, cost effective treatment must be available that results in significantly better outcomes when used before symptoms ever develop. Using these guidelines, several tests have made it into the newborn screen.