Alzheimer's Plasma Extension

The goal of this observational study is to learn how Alzheimer's disease-related blood biomarkers, brain amyloid, and memory and thinking abilities change over time. The study will include up to 3,000 people who previously did not qualify for an anti-amyloid Alzheimer's disease prevention trial based on their amyloid eligibility. Participants may have previously screened for the AHEAD 3-45 Study, A4 Study, or another anti-amyloid Alzheimer's disease prevention trial. The main questions this study aims to answer are: * Can Alzheimer's disease-related blood tests help predict future buildup of amyloid in the brain? * How are changes in Alzheimer's disease-related blood biomarkers related to changes in brain amyloid, memory, thinking, and self-reported cognitive function? * Do these changes differ based on factors such as sex, APOE genetic status, race, ethnicity, socioeconomic status, or social factors? Participants will be followed for about 6 years. They will complete memory and thinking assessments and questionnaires and provide blood samples. Some participants may also be asked to take part in optional activities, including brain imaging with positron emission tomography (PET) or magnetic resonance imaging (MRI), digital cognitive assessments, and questionnaires about habits and social factors.