Diagnosis in relationship to treatment of hyperphenylalaninaemia.

PKU is not a single simply defined entity. It is part of a spectrum of the hyperphenylalaninaemias. Natural protein loading studies with uniform Phe equivalents are simple, and they are an inexpensive and safe way to determine or categorize the types of hyperphenylalaninaemias (excluding defects of biopterin). Evidence from the US PKU Collaborative Study indicates that all patients with PKU do not require indefinite or prolonged restrictive dietary therapy to maintain normal intellectual functioning. Although there are as yet no absolute criteria, it appears that the milder forms of PKU may need treatment for a shorter period of time.