NEFL and Guillain-Barre syndrome: Previously, both NfL and neurofilament heavy chain (NfH) were shown to be elevated in the CSF of patients with GBS, and higher concentrations were associated with clinical outcome.10, 11, 12 Serum NfL levels have been reported to be elevated at baseline in patients with GBS compared to age-matched healthy controls and were associated with disease severity.13