Atypical phenylketonuria (PKU) is caused by tetrahydrobiopterin (BH4) deficiency. In Italy a systematic screening service for BH4 deficiency is not currently performed, therefore its real frequency is not known yet. We determined urinary excretion of biopterin (B) and neopterin (N) by HPLC in 74 phenylketonuric and hyperphenylalaninemic subjects, including all newborns with positive Guthrie's test for PKU since 1984. We found two patients with Nand B urinary values above the classical PKU

more »

... In these subjects we also performed a BH410ading test that restored a normal plasma phenylalanine level in one case, while it had no effect in the other one. In both of them, the enzymatic activity of dihydropteridine reductase on dried blood spots was absent. We conclude that the determination of urinary pteridines together with the BH4 loading test are useful and informative tools in the screening of BH4 deficiencies, but that a conclusive diagnosis can only be achieved by the enzymatic assays.