Introduction: Commonly occurring dementias include those of Alzheimer’s, vascular, and mixtures of these and other pathologies. They are believed to evolve over many years, but that time interval has been difficult to establish. Our objective was to determine how many years in advance of a dementia diagnosis cognitive scores begin to change. Methods: 14,086 dementia-free ARIC participants underwent a cognitive exam at baseline visit 2 (1990–1992, mean age 57 ± 5.72), and 11,244 at visit 4 (1996–1998), 5,640 at visit 5 (2011–2013), and 3,574 at visit 6 (2016–2017) with surveillance for dementias of all-causes combined. Within 5-year intervals after each visit, we compared performance on the Delayed Word Recall Test (DWRT), the Digit Symbol Substitution Test (DSST), the Word Fluency Test (WFT), and the combined mean of three cognitive tests at baseline in participants who were diagnosed with dementia within each interval versus those who survived the interval without a dementia diagnosis. Z-scores were adjusted for demographics and education in separate regression models for each visit. We plotted adjusted z-score means by time interval following each visit. Results: During follow-up 3,334, 2,821, 1,218, and 329 dementia cases were ascertained after visits 2, 4, 5, and 6, respectively. Adjusted DWRT z-scores were significantly lower 20–25 years before dementia than those who did not experience dementia within 25 years. DSST z-scores were significantly lower at 25–30 years and 3-test combination z-scores were significantly lower as early as 30–31 years before onset. The difference between dementia and non-dementia group in the visit 2 3-test combination z-score was −0.20 at 30–31 years prior to dementia diagnosis. As expected, differences between the dementia and non-dementia groups increased closer to the time of dementia occurrence, up to their widest point at 0–5 years prior to dementia diagnosis. The difference between dementia and non-dementia groups in the visit 2 3-test combination z-score at 0–5 years was −0.90. WFT z-score differences were smaller than for the DSST or DWRT and began later. Patterns were similar in Black and White participants. Conclusion: DWRT, DSST, and combined 3-test z-scores were significantly lower more than 20 years prior to diagnosis in the dementia group versus the non-dementia group. Findings contribute to our knowledge of the long prodromal period in Blacks and Whites.

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