APOE and Alzheimer disease: Here we assessed the clinical performance of using the Aβ42/40 ratio to identify amyloid PET status in a well-characterized cohort that included age-matched Aβ PET-positive and Aβ PET-negative individuals characterized as AD, MCI, or healthy controls with APOE genotype inferred from apolipoprotein E (ApoE) proteoform.