Guthrie test /guth′re/ , a screening tool used with infants to determine the level of phenylalanine in the blood. Blood is placed on filter paper, which is then placed on agar plates with a strain of Bacillus subtilis that requires phenylalanine for growth. If there is excessive phenylalanine in the blood sample, a halo will form around the filter paper, and additional tests are required to determine the seriousness of the hyperphenylalaninemia.
