NEFL and Parkinson disease: Furthermore, the NF-L levels increased over time and with age (by 3.35% per year of age, and women had a median serum level 6.79%, which was higher than that of the men), were correlated with PD severity and, most importantly, were highest in early PD, but also relatively high in the prodromal groups, including patients affected by RBD disorders, suggesting a prognostic role.