Phenylalanine Hydroxylase Activity

Sir.—In the article, "Diagnosis of Phenylalanine Hydroxylase Deficiency (Phenylketonuria)" (Journal 1982;136:111-114), we are concerned by the recommendation of Berry and collegues that centers treating patients with phenylketonuria (PKU) should " . . . consider measurement of PH [phenylalanine hydroxylase] activity a necessary factor in the diagnosis of PKU." They claim that measurement may provide information about the "degree of dietary control required for optimum intellectual development" and "the selection of patients for dietary discontinuation."

Although we do not know the mechanism of brain damage in PKU, there is abundant evidence that it is related to the serum phenylalanine levels in infancy and that careful dietary control of this level allows good brain development.¹ It is difficult to see how PH activity measured in vitro can predict intellectual development, except by predicting the serum phenylalanine level and/or the tolerance of dietary phenylalanine. The serum phenylalanine level is easily measured, and dietary tolerance is