Inclusion Criteria:  Patients aged between 20 and 80 years and whose laboratory results included the levels of serum Cystatin C within a month from the PET-DAT were recruited.  Exclusion Criteria:  To make the study cohort more homogeneous, PD patients with either a long-standing duration (≥5 years) or advanced Hoehn and Yahr (H&Y) staging (≥3) were excluded.