PROJECT SUMMARY Amyloid-beta (Aβ) has been shown to be critical in the pathophysiology of Alzheimer's disease (AD). Cerebrospinal fluid (CSF) Aβ and amyloid positron emission tomography (PET) tracers are established biomarkers for detecting amyloid plaques in the brain. Current screening for clinical trials, especially prevention trials, are hampered by complex, expensive or invasive procedures, with limited availability. Our overall goal is to determine the relationship between blood plasma Aβ and the presence of amyloid plaques in the human brain to improve screening for AD trials and to validate an AD blood test. The relationship and timing of blood Aβ changes relative to amyloid PET measures of plaques and CSF Aβ are not understood. There are several questions that have not been addressed before: ‘Does blood Aβ change before, at the same time or after changes in CSF or amyloid PET?’, ‘What demographic, clinical, or genetic factors influence plasma Aβ?‘. We hypothesize that abnormal plasma Aβ is predictive of abnormal CSF Aβ and amyloid PET. In Aim 1, we will leverage existing plasma and CSF samples from regional, national and international studies to test our hypothesis in well-characterized research cohorts. We will use longitudinal samples to investigate the temporal sequence of changes of plasma Aβ, CSF Aβ, and amyloid PET to understand at which stage of AD plasma Aβ becomes abnormal. We will also look at several cofactors to determine their impact on plasma Aβ measures of brain amyloid plaques, including age and APOE ϵ4. In Aim 2, we will implement a cross-sectional clinical study in diverse community and clinic-based populations, using the blood Aβ test to screen individuals for amyloidosis with confirmation using amyloid PET. Based on the results of the blood amyloid test, a subset of participants will follow-up with confirmatory amyloid PET and repeat blood collection and tests. This will allow us to compare the blood Aβ test to the current gold standard of amyloid PET scans. The proposed work builds on the prior pioneering approach that has influenced the understanding of the role of Aβ in the amyloid hypothesis and pathophysiological causes of AD. We have extended this approach with significantly improved techniques, and discovery of a blood Aβ signature of amyloid plaques. We now seek to validate the test in a real-world study with a diverse observational study. These studies will lay the groundwork for rapid deployment to accelerate enrollment of AD trials and clinical diagnosis.