NEFL and polyneuropathy: Six studies found a correlation between NfL levels and disease severity measured by the (modified + 7) Neuropathy Impairment Score (NIS) [23,24,35,39,40,41], six studies between NfL levels and disease severity measured by the polyneuropathy disability (PND) score [24,25,33,39,41,42], and three studies between NfL and the FAP stage [24,39,41].