NEFL and central nervous system disorder: As MS is primarily a CNS (central nervous system) disease and ALS usually involves motorneurons in both the CNS and the PNS (peripheral nervous system), but in contrast to GBS, has essentially preserved integrity of the BCSFB, we hypothesized that by calculating the NfL ratio and NfL index in these study cohorts, we could determine the source of NfL and the effect of a damaged BCSFB on NfL levels.