Answer
It needs enough remaining phenylalanine-hydroxylase activity to improve. Responsiveness is confirmed by monitoring the change in blood phenylalanine.
Sources for this answer
- Sapropterin Dihydrochloride 100mg Soluble Tablets — Current selected SmPCemc · Summary of Product Characteristics
- NHSNHS · NHS medicine guidance
General medicine information. Follow the instructions for your own medicine and speak to a pharmacist or clinician about your individual circumstances.