Lower Odor Identification in Subjective Cognitive Decline: A Meta-analysis.

Introduction: Odor identification correlates with Alzheimer's disease (AD) biomarkers, and its decline may emerge before measurable cognitive deficits-as early as the subjective cognitive decline (SCD) stage. We aimed to compare odor identification between SCD and cognitively normal (CN) stages and investigate whether cognitive differences moderate olfactory deficits.

Methods: A systematic search of four databases identified studies assessing olfactory identification and cognitive screening in individuals aged 50+. A random-effects meta-analysis was performed on 11 studies (660 SCD, 574 CN).

Results: Individuals with SCD exhibited lower olfactory identification scores compared to CN participants (SMD = -0.67, 95%CI [-1.31, -0.03], p = .04). Meta-regression revealed a negative association (β = -1.79, p = .02) between cognitive and olfactory differences, indicating that greater cognitive decline was not consistently associated with greater olfactory deficits, lower odor identification scores in SCD occurred despite minimal cognitive differences across groups.

Discussion: Odor identification is lower in pre-MCI individuals reporting SCD. Olfactory decline may emerge independently prior to measurable cognitive decline, supporting the role of odor identification as a screen for AD.