MAPT and early-onset autosomal dominant Alzheimer disease: In clinically and racially diverse training and test sets, we evaluated and validated Alzheimer's disease (AD) plasma biomarkers: phosphorylated tau‐217 (p‐tau217), amyloid beta 1‐42/1‐40 (Aβ42/Aβ40), and p‐tau217/Aβ42.