QDPR and phenylketonuria: When patients with PKU and related disorders are identified via NBS, their clinical phenotypes are classified as follows: (1) tetrahydrobiopterin (BH4) deficiency is ruled out by pteridine analysis and measurement of dihydropteridine reductase activity, (2) PAH deficiency is classified by pretreatment-Phe levels, and (3) Patients with <60 μmol/L Phe level are diagnosed with HPA while those with >360 μmol/L are further classified as BH4-responsive mild PKU or classical PKU based on BH4 loading test.