Plasma phenylalanine and tyrosine responses to different nutritional conditions (fasting/postprandial) in patients with phenylketonuria: effect of sample timing.

OBJECTIVE: To evaluate the adequacy of dietary treatment in patients with phenylketonuria, the monitoring of plasma phenylalanine and tyrosine concentrations is of great importance. The preferable time of blood sampling in relation to the nutritional condition during the day, however, is not known. It was the aim of this study to define guidelines for the timing of blood sampling with a minimal burden for the patient. DESIGN: Plasma concentrations of phenylalanine and tyrosine were measured in nine patients with phenylketonuria who had no clinical evidence of tyrosine deficiency. These values were measured during the day both after a prolonged overnight fast, and before and after breakfast. RESULTS: Phenylalanine showed a small rise during prolonged fasting, while tyrosine decreased slightly. After an individually tailored breakfast, phenylalanine remained stable, while tyrosine showed large fluctuations. CONCLUSION: It is concluded that the patient's nutritional condition (fasting/postprandial) is not important in the evaluation of the phenylalanine intake. To detect a possible tyrosine deficiency, however, a single blood sample is not sufficient and a combination of a preprandial and postprandial blood sample on the same day is advocated.