The Simoa Human Neurology 3-Plex A assay (N3PA) measures 3 major neurology biomarkers in both cerebrospinal fluid (CSF) and blood. The 3 targets are total tau, amyloid β 1-42, and amyloid β 1-40. Tau is a microtubule-stabilizing protein primarily localized in central nervous system neurons but also expressed at low levels in astrocytes and oligodendrocytes. Tau consists of six isoforms in the human brain with molecular weights of 48,000 to 67,000 daltons, depending on isoform. Aβ42 and Aβ40 are two proteolytic products from the amyloid precursor protein (APP). Beta-secretase cleavage of APP initially results in the production of an APP fragment that is further cleaved by gamma-secretase at residues 40 or 42 to generate two main forms of amyloid beta, Aβ42 and Aβ40. Amyloid beta (Aβ) peptides (including a shorter Aβ38 isoform) are produced by different cell types in the body, but the expression is particularly high in the brain. Tau and amyloid β related pathologies have been the hallmark of Alzheimer’s disease. CSF and blood tau and amyloid have been tested and monitored as potential biomarkers for Alzheimer’s disease, mild cognitive impairment, vascular dementia, and other neurodegenerative disorders.