Newborn screening is a public health program designed to screen infants shortly after birth for a list of conditions that are treatable, but not clinically evident in the newborn period. Some of the conditions included in newborn screening programs are only detectable after irreversible damage has been done, in some cases sudden death is the first manifestation of the disease. Screening programs are often run by state or national governing bodies with the goal of screening all infants born in the jurisdiction. The number of diseases screened for is set by each jurisdiction, and can vary greatly. Most newborn screening tests are done by measuring metabolites and enzyme activity in whole blood samples collected on specialized filter paper, however many areas are starting to screen infants for hearing loss using automated auditory brainstem response and congenital heart defects using pulse oximetry. Infants who screen positive undergo further testing to determine if they are truly affected with a disease or if the test result was a false positive. Follow-up testing is typically coordinated between geneticists and the infant's pediatrician or primary care physician.Newborn screening debuted as a public health program in the United States in the early 1960s, and has expanded to countries around the world, with different testing menus in each country. The first disorder detected by modern newborn screening programs was phenylketonuria, a metabolic condition in which the inability to degrade the essential amino acid phenylalanine can cause irreversible mental retardation unless detected early. With early detection and dietary management, the negative effects of the disease can be largely eliminated. Robert Guthrie developed a simple method using a bacterial inhibition assay that could detect high levels of phenylalanine in blood shortly after a baby was born. Guthrie also pioneered the collection of blood on filter paper which could be easily transported, recognizing the need for a simple system if the screening was going to be done on a large scale. Newborn screening around the world is still done using similar filter paper.