Robert Guthrie, MD, Ph.D. (June 28, 1916 – June 24, 1995) was an American microbiologist, best known for developing the bacterial inhibition assay used to screen infants for phenylketonuria at birth, before the development of irreversible neurological damage. Guthrie also pioneered the collection of whole blood on specially designed filter paper, commonly known as "Guthrie cards" as a sample medium that could be easily collected, transported and tested. Although Guthrie is best known for developing the test for phenylketonuria, he worked tirelessly to raise awareness of the need to screen for treatable conditions and adapted his method to early screening tests for galactosemia and maple syrup urine disease. Early life Guthrie received his doctorate from the University of Minnesota, although his education took a circuitous route, as he eventually earned six degrees in six years, including both a medical doctorate and a doctor of philosophy. While in school, Guthrie married Margaret, a fellow student, and they eventually had six children together. His early research into bacterial inhibition assays came while he was employed by the Staten Island Public Health Hospital, testing antibiotic sensitivity. Research interests Guthrie became interested in causes and prevention of mental retardation after his son, John, was born disabled in 1947. Despite his work in the field, the cause of his son's disability was never diagnosed. In 1958, Guthrie's 15-month-old niece was diagnosed with phenylketonuria (PKU), a condition in which the body cannot metabolize phenylalanine. Untreated PKU results in irreversible neurological damage. After the discovery of PKU as a cause of mental retardation, Horst Bickel and colleagues discovered that it could be treated successfully with a diet low in phenylalanine. The main drawback in successful treatment of PKU was the delay in identifying affected individuals. The common test for PKU at the time was mixing urine with ferric chloride. The excess phenylpyruvic acid in the urine of an individual with PKU would produce a bright green colour when reacting with the ferric chloride. Infants do not excrete high enough concentrations of this compound to give a positive test result, thus delaying their diagnosis, and allowing irreversible damage to take place. Guthrie's disabled son had driven his interest in causes of mental retardation, the diagnosis of his niece with PKU turned his attention to preventable causes. Others working with children who had PKU asked Guthrie to focus on a test that would allow for earlier identification, before irreversible