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Background: Depression is both a risk factor for and early symptom of Alzheimer’s disease. Numerous studies have investigated the relationship between AT(N) biomarkers of AD and depression; however, the majority of these have utilized CSF in Non-Hispanic White populations.

Objective: To investigate the relationship between plasma total Tau, Aβ₄₀, Aβ_42, NFL and depression in Mexican-Americans and Non-Hispanic Whites.

Methods: The study was a cross-sectional comparison of 645 Mexican American and 644 Non-Hispanic White older adults in a community-based study of cognitive aging who had been categorized as having unimpaired cognition using a consensus based algorithmic approach. Plasma biomarkers were assayed using Simoa technology. The Geriatric Depression Scale assessed depression.

Results: Mexican Americans had significantly higher scores on the GDS. Non-Hispanic Whites had higher Aβ₄₀, and Aβ₄₂ and MAs had higher NFL and Tau. For Mexican Americans, linear regression analyses found NfL and Aβ₄₂ significant predictors of GDS scores whereas for the Non-Hispanic White group none of the biomarkers was significantly related to GDS total score or any of the subscale scores. Those scoring in the depressed range had significantly higher levels of Aβ₄₀, Aβ_42, and NFL. When analyzed by ethnicity the depressed Mexican Americans had significantly higher levels of Aβ₄₀, Aβ_42, and NFL than the non-depressed. No difference between the depression levels on any of the biomarkers was found for Non-Hispanic Whites.

Conclusions: Findings support the importance of evaluating the effect of ethnicity and level of depressive symptoms when assessing the relationship of AD biomarkers to depression. In cognitively unimpaired MAs depression is related to the Alzheimer’s Disease biomarkers but this is not the case for Non-Hispanic Whites. Higher levels of these biomarkers among depressed cognitively unimpaired Mexican Americans may be an indicator of increased risk for cognitive impairment but not for Non-Hispanic Whites. Longitudinal research is needed to clarify the effect of ethnicity on the biomarker-depression relationship.