Alzheimer's disease (AD) is characterized by the accumulation of tau pathology, and blood-based biomarkers such as phosphorylated tau-217 (pTau217) have been identified as sensitive and specific predictors of AD risk. Recent studies suggest that individuals with elevated pTau217 levels may be at increased risk for developing AD and cognitive dysfunction. This observational study will examine donated human plasma samples to determine whether some units of donated blood contain abnormally elevated pTau217 concentrations. The overarching goal is to evaluate whether transfusion of blood with higher pTau217 may pose risks to recipients and whether such units should be avoided in clinical use.