APOE and Alzheimer disease: Therefore, in this population-based study of rural-dwelling older adults, we aimed to 1) describe the distribution of plasma Aβ, t-tau, and NfL by demographics and apolipoprotein E (APOE) genotype; 2) examine the associations of these biomarkers with the clinical spectrum from normal cognition through MCI to clinical AD; and 3) evaluate the performance of these biomarkers to differentiate individuals with normal cognition, MCI, and AD.