Alzheimer's Plasma Extension

Inclusion Criteria: 1. Documentation of the participant's informed consent to study procedures and for the use of protected health information (HIPAA Authorization, or local, regional or country-specific equivalent). Informed consent processes and documentation must adhere to state laws/local requirements, including consent provided by the participant's legally authorized representative (LAR), responsible next of kin, surrogate consent with assent, etc. 2. Previously consented to participate in an anti-amyloid preclinical AD study (e.g., AHEAD 3-45, A4 or other anti-amyloid preclinical AD study). 3. Has screening results that did not meet amyloid eligibility criteria for an anti-amyloid preclinical AD trial (e.g., AHEAD 3-45, A4, or other anti-amyloid preclinical AD study). 4. As assessed by the site PI, participant is likely to be able to comply with the protocol, including completion of all required procedures for the duration of the study, and has adequate vision, hearing (hearing aid permitted), and literacy in English or Spanish sufficient for compliance with required testing procedures. For participants who screen failed from the AHEAD 3-45 Study: 5. Has AHEAD 3-45 screening plasma biomarker results required for determining eligibility to participate in the AHEAD 3-45 Study. Exclusion Criteria: 1. Current treatment with an FDA approved medication for Alzheimer's disease, including prior or current treatment with a prohibited medication. 2. Enrollment in another investigational study, or intake of investigational drug, within 30 days prior to screening, or five half-lives of the investigational drug, whichever is longer, unless it can be documented that the participant was in the placebo treatment arm. 3. Screen failed from an anti-amyloid preclinical AD trial (e.g. AHEAD 3-45, A4 or other anti-amyloid preclinical AD trial) due to not meeting basic inclusion criteria (i.e., age requirement; current diagnosis of AD dementia). For participants participating in the optional amyloid PET imaging sub-study: 4. Is pregnant or breastfeeding.