The two proteinopathies that define Alzheimer's disease—deposits of aggregated amyloid β and deposits that contain a mixture of three-repeat (3R) and four-repeat (4R) tau isoforms—can be detected in vivo. ...recently, however, the detection of these biomarkers has been either expensive, in the case of PET imaging, or invasive, in the case of CSF sampling, which requires a lumbar puncture. Because plasma p-tau181 seems to indicate occurrence of both amyloid β deposition and tauopathy in Alzheimer's disease, it could be used in trials that target amyloid β or tau or both. To date, a diagnosis of Alzheimer's disease based on biological grounds was impractical in most clinical settings. Because plasma p-tau181 seems to be an indicator of both amyloid β and tauopathy, a single, non-invasive assay could be done to determine whether an individual is on the Alzheimer's disease pathophysiological pathway.