Estimates of age-related memory decline are inflated by unrecognized Alzheimer's disease.

Cognitive decline is considered an inevitable consequence of aging; however, estimates of cognitive aging may be influenced negatively by undetected preclinical Alzheimer's disease (AD). This study aimed to determine the extent to which estimates of cognitive aging were biased by preclinical AD. Cognitively normal older adults (n = 494) with amyloid-β status determined from positron emission tomography neuroimaging underwent serial neuropsychological assessment at 18-month intervals over 72 months. Estimates of the effects of age on verbal memory, working memory, executive function, and processing speed were derived using linear mixed models. The presence of preclinical AD and clinical progression to mild cognitive impairment or dementia during the study were then added to these models as covariates. Initially, age was associated with decline across all 4 cognitive domains. With the effects of elevated amyloid-β and clinical progression controlled, age was no longer associated with decline in verbal or working memory. However, the magnitude of decline was reduced only slightly for executive function and was unchanged for processing speed. Thus, considered together, the results of the study indicate that undetected preclinical AD negatively biases estimates of age-related cognitive decline for verbal and working memory.