MAPT and Alzheimer disease: These markers are increasingly close to integration into clinical pathways in AD: for example, early reductions in the amyloid-β (Aβ) 42:40 ratio and elevation in phospho-tau species (eg, phosphorylated at serine 181, p-tau181 and tau phosphorylated at threonine 217, p-tau217) are sensitive to amyloid pathology.14 15 Neurodegeneration may be indexed by increased levels of total tau, or the axonal marker plasma neurofilament light (NfL).