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Background To date there are no biomarkers with proven reliability as a measure of disease burden in amyotrophic lateral sclerosis ( ALS). The aim of our study is to assess the neurofilament light chain ( NFL) in cerebrospinal fluid ( CSF) samples as a measure of disease activity and progression in ALS. Methods Thirty-seven consecutive patients with ALS, 25 with chronic inflammatory demyelinating polyneuropathy and 21 with other neurodegenerative diseases were evaluated. CSF NFL levels were assayed by two-site solid-phase sandwich ELISA. In patients with ALS, neurological status was assessed by the revised ALS Functional Rating Scale ( ALSFRS-r) and the Medical Research Council scale, and the progression of the disease was evaluated using the 'diagnostic delay' and the 'progression rate'. Results Cerebrospinal fluid NFL levels were higher in ALS cases than in controls ( P < 0.0001). Using receiver operating curve analysis, an optimal NFL cut-off of 1981 ng/l discriminated between patients with ALS and neurological controls, with a sensitivity of 78.4% and specificity of 72.5%. Multivariate logistic regression confirmed the association between CSF NFL levels and the presence of ALS (age and sex adjusted odds ratio for ALS 8.9; 95% CI 3.1-25.8; P < 0.0001). In ALS, CSF NFL negatively correlated with the diagnostic delay ( P < 0.0001) and the ALSFRS-r ( P = 0.014) and positively with the progression rate ( P < 0.0001). Conclusions High CSF NFL levels were found in patients with ALS, reflecting the burden of neurodegeneration. The significant relation between CSF NFL levels and disease progression suggests that NFL may be a useful marker of disease activity and progression in ALS. [ABSTRACT FROM AUTHOR]