NfL (Neurofilament light chain)

Neurofilament light chain is a cytoskeletal protein of large myelinated axons released into cerebrospinal fluid and blood upon neuroaxonal injury. Ultrasensitive single-molecule assays such as Simoa (Quanterix) made serum and plasma NfL quantifiable at picogram-per-millilitre concentrations, transforming it into a disease-agnostic marker of axonal damage. Concentrations rise with age and across multiple sclerosis, amyotrophic lateral sclerosis, Alzheimer disease, frontotemporal dementia, traumatic brain injury and stroke. In multiple sclerosis it tracks relapse activity and treatment response; in ALS it correlates with progression and survival; in Alzheimer disease it indexes neurodegeneration within the ATN(N) framework. NfL is non-specific for aetiology and must be interpreted with age-adjusted reference values, but longitudinal change within an individual carries prognostic information.