MODEL RELEASED. Guthrie blood test. Doctor taking a blood sample from the foot of a newborn baby boy. This is the Guthrie test, carried out on newborn babies to detect phenylketonuria, a rare inherited disorder of amino acid metabolism. Before this test was developed in 1963, the condition caused brain damage and mental retardation in undiagnosed cases. Screening programs, such as the one seen here, were set up to prevent this. If the test is positive, further tests are needed. Patients with phenylketonuria manage the disorder by eating a diet low in phenylalanine.