Alzheimer's Plasma Extension (APEX Trial)

The APEX study is a multicenter, observational study designed to capture longitudinal follow-up of plasma biomarkers and cognitive and functional assessments on individuals who screen failed in the AHEAD study over approximately 4 years. Approximately 1000 participants will be enrolled across three groups: * Group A: Approximately 500 participants who are discordant on screening (plasma positive / Positron Emission Tomography (PET) negative), * Group B: Approximately 250 participants who are concordant on screening (plasma negative / PET negative), and * Group C: Approximately 250 participants selected from the individuals who previously screen failed prior to PET for the AHEAD study with oversampling of racial and ethnic populations underrepresented in Alzheimer's disease (AD) clinical trials. Primary Objectives: * Collect longitudinal cognitive and functional assessments and blood-based biomarker data * Evaluate, characterize, and compare the longitudinal cognitive and functional data between the three groups of participants * Compare longitudinal change across race and ethnicity, sex, and Apolipoprotein E (ApoE) status Exploratory Objectives: • Collect baseline amyloid PET on participants without prior amyloid PET data (Group C)