damage had taken place. The Guthrie test Guthrie developed a simple method to screen for elevated phenylalanine levels using a bacterial inhibition assay. He cultured Bacillus subtilis on agar in the presence of a phenylalanine antagonist, inhibiting the growth. When exposed to blood from patients affected with PKU, the high levels of phenylalanine overcame the inhibition, and bacterial growth was visible. This assay was initially developed to allow monitoring of phenylalanine concentrations in known patients on dietary treatment using serum spotted onto filter paper. Guthrie recognized both the utility of this method as a screening test, and the need to eliminate serum as the sample type to minimize processing. He tested the assay using whole blood collected on filter paper from a heel stick. The collection of whole blood on special filter paper developed by Guthrie is still used in newborn screening programs around the world, allowing babies to be screened shortly after birth for a number of treatable conditions. After establishing a test that could identify PKU in whole blood spots, Guthrie set out to test his method, starting in an institution in New York. Here, his test correctly identified all patients known to have PKU and also four who had previously been undiagnosed. In 1961, Guthrie and his lab started screening infants for PKU, a project that quickly expanded. In two years, they had tested 400,000 American newborns, and diagnosed 39 with PKU. This early diagnosis allowed for early treatment and avoidance of the most severe consequences of the disease. Throughout the 1960s, PKU testing expanded in the United States and around the world, eventually becoming required by law in many jurisdictions. With the success of PKU testing, Guthrie and his colleagues focused on screening tests for other diseases that can affect newborns. They developed bacterial inhibition assays for galactosemia and maple syrup urine disease that could be run using the same sample collection as the PKU test. Patent controversy Guthrie decided that commercial production would be the most efficient way to manufacture 400,000 test kits, so he approached the Ames Company, a division of Miles Laboratories, which manufactured the older PKU tests. Ames said it would only manufacture the kits if a patent was issued, so Guthrie filed a patent application in 1962 and signed an exclusive licensing agreement with Miles, under which he would receive no royalties and 5% of the proceeds would be divided among