NEFL and Alzheimer disease: Blood-based biomarkers offer a less invasive and expensive way to assess AD-related pathology, enabling easier repetition and greater feasibility for longitudinal monitoring.3,9,10 Blood-based biomarkers for AD include phosphorylated tau 217 (pTau217), reflecting both amyloid and tau pathology; amyloid-β42/40 ratio (Aβ42/40), reflecting amyloid pathology; glial fibrillary acidic protein (GFAP), reflecting reactive astrocytosis; and neurofilament light chain (NfL), reflecting neuroaxonal injury.10,11,12,13