Neurofilament Light Chain as a Biomarker in Neurology

BACKGROUND: Neurofilament light chain (NfL) has emerged as a highly sensitive biomarker of neuroaxonal injury across diverse neurological disorders. This narrative review synthesizes current evidence regarding its diagnostic, prognostic, and therapeutic-monitoring utility, while outlining major clinical limitations and emphasizing the complementary role of glial fibrillary acidic protein (GFAP). SUMMARY: NfL concentrations increase following axonal damage and correlate with inflammatory activity, lesion burden, and long-term disability progression in multiple sclerosis. Elevated levels also reflect neurodegeneration in Alzheimer's disease, predict disease severity and survival in amyotrophic lateral sclerosis, and are associated with motor and cognitive decline in Parkinson's disease and multiple system atrophy. In acute neurological conditions, including traumatic brain injury and stroke, NfL serves as a robust indicator of the extent of neuronal injury. Interpretation is constrained, however, by substantial physiological variability related to age, renal function, body mass index, and comorbidities, limiting the utility of absolute cut-off values. GFAP provides complementary information by capturing astrocytic damage, and the GFAP/NfL ratio may aid in differentiating multiple sclerosis from neuromyelitis optica spectrum disorder. Integration of NfL with multimodal biomarkers-such as GFAP, tau proteins, proteomic and metabolomic signatures, and advanced neuroimaging-may enhance diagnostic specificity and prognostic accuracy. KEY MESSAGES: Future research priorities include establishing age-adjusted reference intervals, validating longitudinal thresholds, and incorporating NfL into therapeutic monitoring frameworks. Advances in these areas are expected to improve diagnostic precision and support broader clinical implementation of NfL.