Alzheimer's disease is the most common cause of dementia and currently has no disease-modifying treatments or simple, accurate diagnostic tests. Amyloid beta builds up in the brain and forms plaques in people with Alzheimer's disease, and the buildup of this protein is found decades before symptoms of dementia begin. A blood test may be able to quickly and inexpensively screen people for Alzheimer's disease clinical trials and eventually diagnose Alzheimer's disease in the clinic.