Phenylalanine and tyrosine levels in newborn screening blood samples.

A previously described difference in newborn blood phenylalanine concentrations between those living in urban and non-urban areas in the south west of England has been confirmed and shown to be independent of the type of feed. Several factors including the place of abode, type of feed, birthweight, and the accuracy of the test have been found to affect the measured, phenylalanine concentration in the newborn screening blood spot, and the importance of these results to screening practice is considered. Blood tyrosine also varied with the above factors, but severe, neonatal tyrosinaemia was shown to be a rare problem.