Plasma Neurofilament Light Chain as a Complementary Biomarker for Alzheimer's Disease and Mild Cognitive Impairment: A Systematic Review and Meta-Analysis.

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cognitive decline and memory loss, ultimately leading to a loss of independence and reduced quality of life. As current treatments are most effective in the early stages, reliable blood-based biomarkers are required for early diagnosis, therapeutic monitoring, and disease staging. Plasma neurofilament light chain (NfL), a marker of neuroaxonal injury, is a candidate biomarker; its value relative to established amyloid- and tau-based biomarkers is unclear. A systematic review and meta-analysis comparing plasma NfL with Aβ42, Aβ40, the Aβ42/Aβ40 ratio, and p-tau217 across cognitively normal (control, Con), mild cognitive impairment (MCI), and AD groups was conducted. Fourteen studies that met specific criteria were included. Plasma NfL and p-tau217 increased across disease stages. Aβ42 and Aβ42/Aβ40 were lower in AD/MCI than in Con; Aβ40 was similar across groups. Heterogeneity (I2 < 99%) was observed across biomarkers. Plasma NfL reliably reflected neuroaxonal injury and disease severity from the earliest clinically detectable stage of AD. Plasma NfL may be most valuable when combined with p-tau217/Aβ-based assays to provide a more comprehensive, blood-based assessment of both AD-specific pathology and neurodegeneration. Further studies are needed to validate its prognostic utility and establish clinically applicable cut-off values.