Abstract: A method for detecting p217+tau in blood-based samples from a subject with high sensitivity, accuracy, and precision. The assay comprises contacting a sample with a capture antibody directed against a p217+tau epitope to bind the capture antibody to p217+tau peptides in plasma to form antibody-peptide complexes, and separately contacting the antibody-peptide complexes with a detection antibody to bind the detection antibody to the antibody-peptide complexes. The amount of p217+tau is determined by detecting the detection antibody. The amount of p217+tau detected is used to determine whether the subject has tauopathy or is at risk of developing tauopathy, or whether the subject has amyloidogenic disease or is at risk of developing amyloidogenic disease when the amount of p217+tau peptides is above a predetermined threshold value. The method has improved sensitivity such that the predetermined threshold value is above a Lower Limit of Quantification and/or Lower Limit of Detection of the assay.