### Abstract

**Objective:**

To estimate the prevalence of elevated brain amyloid and reduced cortical thickness (as a marker for neurodegeneration) in a defined population.

**Methods:**

Mayo Clinic Study of Aging participants underwent MRI to assess a composite Alzheimer disease (AD) signature cortical thickness measure and PET to assess brain amyloid accumulation. Participants were characterized as having elevated amyloid (A+/A−), reduced cortical thickness (N+/N−), and A+N+, A+N−, A−N+, or A−N−. The prevalence of AD biomarkers was derived by adjusting for nonparticipation and standardizing to the Olmsted County, Minnesota, population.

**Results:**

Among 1,646 participants without dementia (mean age 70.8 years; 53.2% men), the prevalence (95% confidence interval) of amyloidosis was 21.1% (19.1%–23.2%): women, 24.3%; men, 17.5%. The prevalence of reduced cortical thickness was 28.9% (26.4%–31.5%): women, 27.9%; men, 30.2%. The prevalence estimates of biomarker categories were as follows: A−N−: 61.4%; A+N−: 9.7%; A−N+: 17.4%; and A+N+: 11.5%, and varied by sex and by APOE ε4 carrier status. In men, prevalence estimates were as follows: A−N−: 62.6%; A+N−: 7.3%; A−N+: 19.9%; and A+N+: 10.2%. In women, prevalence estimates were as follows: A−N−: 60.4%; A+N−: 11.7%; A−N+: 15.3%; and A+N+: 12.6%. In ε4 carriers, prevalence estimates were as follows: A−N−: 54.6%; A+N−: 16.6%; A−N+: 12.4%; and A+N+: 16.4%. In non-ε4 carriers, prevalence estimates were as follows: A−N−: 63.3%; A+N−: 6.9%; A−N+: 19.9%; and A+N+: 10.0%.

**Conclusions:**

These prevalence estimates are important for understanding age-related trends in amyloid positivity and AD signature cortical thickness in the population, and for potentially projecting the future burden of biomarkers in elderly persons.