Olfactory identification and incidence of mild cognitive impairment in older age.

CONTEXT: Mild cognitive impairment (MCI) is often a precursor to Alzheimer disease, but knowledge about factors that predict its development is limited.
OBJECTIVE: To test the hypothesis that impaired odor identification is related to increased risk of incident MCI.
DESIGN: Longitudinal cohort study.
SETTING: Academic research. PARTICIPANTS: Subjects were 589 community-dwelling older persons without cognitive impairment at study baseline, at which time odor identification was assessed using the 12-item Brief Smell Identification Test (mean +/- SD score, 9.3 +/- 1.9). MAIN OUTCOME MEASURES: Incidence of MCI and rate of decline in cognitive function.
RESULTS: During annual observation of up to 5 years, 177 subjects developed MCI. In a proportional hazards model adjusted for age, sex, and education, odor identification score predicted development of MCI (relative risk, 1.15; 95% confidence interval, 1.07-1.23), with risk increased by 50% in persons with below-average (score of 8 [25th percentile]) compared with above-average (score of 11 [75th percentile]) odor identification scores. Results were not substantially changed in subsequent analyses that controlled for level of cognitive function or disability, presence of stroke, or smoking status at baseline or that required MCI to persist for at least 1 year. Impaired odor identification was also associated with a lower level of global cognition at baseline and with more rapid decline in episodic memory, semantic memory, and perceptual speed.
CONCLUSION: Among older persons without manifest cognitive impairment, difficulty in identifying odors predicts subsequent development of MCI.