[Reference levels for phenylalanine and tyrosine in discs impregnated with blood during analysis by HPLC]

Aromatic amino acids tyrosine and phenylalanine have been measured on paper with dried blood samples, using high performance liquid chromatography (HPLC) in reverse phase. The aim of this procedure is to discriminate unclear cases of general screening for aminoacidopathies avoiding unnecessary retest. Plasma normal values of tyrosine and phenylalanine have been obtained in full term babies (0.82 +/- 0.39 mg/dl and 0.53 +/- 0.23 mg/dl) preschool boy (0.78 +/- 0.21 mg/dl and 0.63 +/- 0.20 mg/dl), school boys (0.89 +/- 0.16 and 0.76 +/- 0.22 mg/dl) and normal adults (1.48 +/- 0.19 and 1.41 +/- 0.12 mg/dl). In order to assess if fasting levels can be altered by breast feeding or formula feeding, a sample was obtained after various feeds and postprandially. Results show no differences before or after feeding. A group of malnourished infants showed greater plasma values of tyrosine and phenylalanine (p less than 0.002) conversely a group children suffering for diabetes showed no differences when comparing with matched age controls. In conclusion, measurement of tyrosine and phenylalanine on dried blood in filter paper is accurate enough, to avoid unnecessary recall in unclear cases of screening, and those levels do not alter significantly with normal milk intake.