In this cohort study, which included CU individuals from 3 independent cohorts (theSwedish BioFINDER-2 study, Knight Alzheimer Disease Research Center, and the SwedishBioFINDER-1 study), a combination of baseline plasma phosphorylated tau 217 (p-tau217)and Aβ42/40 better predicted longitudinal changes in brain Aβ load thanindividual biomarkers in 514 participants with low brain Aβ levels at baseline. Here, MAPT is linked to early-onset autosomal dominant Alzheimer disease.