Odor identification and discrimination as markers of early Alzheimer’s disease

Abstract

AbstractBackgroundReliable markers are essential in order to achieve accurate diagnosis in the early stages of Alzheimer’s disease (AD). Olfactory impairment has been identified as a possible non‐invasive biomarker of AD. Odor identification tasks are most commonly used for measuring olfactory abilities. However, odor identification is associated to verbal abilities and cultural context. The aim of this study was to determine whether odor discrimination could be a valid marker of early AD.Method65 participants were enrolled in the study. 26 patients with mild cognitive impairment due to AD (MCI‐AD), 26 patients with mild dementia due to AD (MD‐AD) and 13 cognitively normal elderly subjects (CN). All participants underwent cognitive examination (MMSE, CDR, ADAS‐Cog 13, fluency tests) and olfactory function assessment (Burghart Sniffin’ Sticks odor identification and odor discrimination tests).ResultMD‐AD and MCI‐AD subjects performed significantly worse than CN subjects in both odor identification (mean correct responses ± SD was 6.77±1.91; 9.08±2.30; 13.31±1.55 respectively; p<0.001) and odor discrimination (mean correct responses ± SD was 6.27±1.66; 8.19±1.65; 12.92±1.71 respectively; p<0.001) tests. MD‐AD subjects performed significantly worse than MCI‐AD subjects in both odor identification (p<0.05) and odor discrimination (p<0.001) tests.Both odor identification and odor discrimination tests had AUC>0.9 on ROC analysis for differentiating MCI‐AD subjects from CN subjects (AUC 0.938 and 0.982) and for differentiating MD‐AD subjects from CN subjects (AUC 0.99 for both). Neither odor identification, nor odor discrimination test reached AUC>0.8 on ROC analysis for differentiating MCI‐AD from MD‐AD subjects (AUC 0.777 and 0.793).Odor identification differentiated MCI‐AD from CN subjects with sensitivity of 69% and specificity of 100% at cut‐off score of ≤10 correct responses. Odor discrimination differentiated MCI‐AD from CN subjects with sensitivity of 77% and specificity of 100% at cut‐off score of ≤9 correct responses.ConclusionThe study confirmed that tests for olfactory dysfunction can reliably differentiate patients with mild cognitive impairment due to AD from cognitively normal subjects. Both odor identification and discrimination are reliable markers for identification of patients with early Alzheimer’s disease.