MAPT and Alzheimer disease: Assays for plasma Aβ fragments [ratio of Aβ1–42 (Aβ42) to Aβ1–40 (Aβ40)] reflect brain amyloidosis4–7; however, these assays have limitations, including the impact of substantial peripheral Aβ production.8 By contrast, CSF and plasma tau phosphorylated at threonine 181 (P-tau181) is a highly specific pathological marker of AD that remains normal in other dementias.9,10 Glial fibrillary acidic protein (GFAP) and neurofilament light chain (NfL) are putative non-amyloid plasma-based biomarkers indicative of ongoing neuroinflammatory and neurodegenerative disease processes.