Editing Neonatal heel prick (section)

==Mechanism==
The test uses the growth of a strain of bacteria on a specially-prepared agar plate as a sign for the presence of high levels of phenylalanine, phenylpyruvate, and/or phenyllactate.  The compound B-2-thienylalanine will inhibit the growth of the bacterium ''Bacillus subtilis'' (ATCC 6051) on minimal culture media.  If phenylalanine, phenylpyruvate, and/or phenyllactate is added to the medium, then growth is restored.  Such compounds will be present in excess in the blood or urine of patients with PKU.  If a suitably-prepared sample of blood or urine is applied to the seeded agar plate, the growth of the bacteria in the test will be a positive indicator for PKU in the patient.<ref name=uic>{{cite web|url=http://www.uic.edu/classes/phar/phar332/Clinical_Cases/aa%20metab%20cases/PKU%20Cases/PHE-TST001.htm |title=PHE-TST001 |work=uic.edu |url-status=dead |archive-url=https://web.archive.org/web/20130927101945/http://www.uic.edu/classes/phar/phar332/Clinical_Cases/aa%20metab%20cases/PKU%20Cases/PHE-TST001.htm |archive-date=2013-09-27 }}</ref>

To prepare the sample for application, a small amount of blood (from a heel puncture, for example) or urine (from a diaper, for example) is applied to a piece of filter paper.  Then a small disc is punched from the center of the spot of blood or urine, and the disc applied to the surface of a seeded, minimal-medium agar plate that contains added beta-2-thienylalanine.  If the sample contains phenylalanine, phenylpyruvate, and/or phenyllactate then these compounds will diffuse into the agar medium.  If their concentrations are high enough (as with the excess levels seen with PKU), bacteria will grow under the disc, but not elsewhere.  Generally an overnight incubation is enough to determine whether phenylalanine, phenylpyruvate, and/or phenyllactate are present in unusual concentrations in blood or urine.<ref name=uic />