Differential diagnosis of hyperphenylalaninaemia by a combined phenylalanine-tetrahydrobiopterin loading test.

[classical phenylketonuria]

We describe a new fully reliable method for the differential diagnosis of tetrahydrobiopterin-dependent hyperphenylalaninaemia (HPA ). The method comprises the combined phenylalanine (Phe ) plus tetrahydrobiopterin (BH 4 ) oral loading test and enables the selective screening of BH 4 deficiency when pterin analysis is not available or when a clear diagnosis has not been previously made . It should be performed together with the measurement of dihydropteridine reductase (DHPR ) activity in blood . The new combined loading test was performed in nine patients with primary HPA , three with classical phenylketonuria (PKU ), three with DHPR deficiency , and three with 6 -pyruvoyl tetrahydropterin synthase (PTPS ) deficiency . Three hours after oral Phe loading (100 mg /kg body weight ), synthetic BH 4 was administered orally at doses of either 7 .5 or 20 mg /kg body weight . Amino acid (Phe and tyrosine ) and pterin (neopterin and biopterin ) metabolism and kinetics were analysed . By exploiting the decrease in serum Phe 4 and 8 h after administration , a clear response was obtained with the higher BH 4 dose (20 mg /kg body weight ), allowing detection of all cases of BH 4 deficiency , as well as differentiation of BH 4 synthesis from regeneration defects . Since DHPR deficient patients who were previously shown to be non-responsive to the simple BH 4 loading test gave a positive response , the combined Phe plus BH 4 loading test can be used as a more reliable tool for the differential diagnosis of HPA in these patients . Moreover , it takes advantage of being performed while patients are on a Phe-restricted diet .