NEFL and amyotrophic lateral sclerosis: When stratified by the region of symptom onset, patients with spinal onset or bulbar onset ALS both showed a higher correlation between plasma NFL and CSF NFL levels (ρ = 0.790, p < 0.0001 and ρ = 0.762, p < 0.0001, respectively) (Fig. 3E) than all ALS patients as a whole group.