Treatment of Classical PKU A low-phenylalanine diet.  Administration of large neutral amino acids (LNAAs) is another approach to diet therapy.  Sapropterin dihydrochloride (Kuvan), a synthetic form of BH4, which acts as a cofactor in patients with residual PAH activity, is approved by the FDA to reduce phenylalanine levels in PKU.  Preliminary trials with recombinant phenylalanine ammonia lyase have been encouraging and demonstrated reduced blood levels of phenylalanine during treatment. Lab Diagnosis of PKU Guthrie Test (Bacterial Inhibition Assay of Guthrie).Rapid screening test in the blood sample. First method used for this purpose.  Key Concept: A low-phenylalanine diet, administration of large neutral amino acids (LNAAs)  and sapropterin dihydrochloride, all these comprise the treatment modalities for Phenylketonuria. Guthrie test is used for its rapid screening. Reference-  Harper’s illustrated biochemistry. 30th edition page -304