Assessment of tetrahydrobiopterin (BH4)-responsiveness and spontaneous phenylalanine reduction in a phenylalanine hydroxylase deficiency population

► 16 of 36 patients represented by 12 of 29 genotypes were proven as BH4 responders. ► A PRA of at least 15.5% was needed for the responder genotype in our population. ► Spontaneous reduction of Phe levels within 24 h was very slight if present.