Phenylketonuria - It is inability of the body to utilize the aminoacid phenylalanine.[1] Methos[1] -A drop of blood is obtained on the second or third day of life. -The blood is collected on a piece of filter paper and is sent to a central laboratory. -A small disk of the filter paper is punched out and placed on an agar gel plate containing Bacillus subtilis and B-2-thienylalanine. The agar gel supports bacterial growth but the B-2- thienylalanine inhibits bacterial growth. - but in the presence of extra phenylalanine, the inhibition is overcome and the bacteria grows. - Within a day the bacterial growth surrounding the paper disk is visible to the eye. -The amount of growth is proportional to the amount of phenylalanine in the serum. Ref: http://www.scribd.com/doc/4448594/Screening-Test-for-Phenylketonuria