Just as different flowers have distinct scents, Alzheimer’s disease and other neurodegenerative disorders can be differentiated based on olfactory deficits. A recent systematic review and meta-analysis explored the potential of smell tests in early and differential diagnosis of Alzheimer’s disease. The study analyzed various smell tests used to assess olfactory functions in patients with Alzheimer’s disease and other neurodegenerative disorders. The findings revealed that smell tests were effective in distinguishing Alzheimer’s disease from mild cognitive impairment, Lewy body disease, depression, and vascular dementia. However, these tests were not as effective in differentiating Alzheimer’s disease from frontotemporal dementia. Interestingly, the University of Pennsylvania Smell Identification Test (UPSIT) was frequently used to discriminate Alzheimer’s disease from mild cognitive impairment, while the Brief Smell Identification Test (B-SIT) was commonly used for Alzheimer’s disease vs. Lewy body disease. The study also identified moderating factors such as age, MMSE scores, and education years that influenced olfactory performance. Overall, this research highlights the potential of smell tests as a valuable tool for early differential diagnosis of Alzheimer’s disease. To learn more about this fascinating investigation, dive into the underlying research!

BackgroundThere are discrepancies of olfactory impairment between Alzheimer’s disease (AD) and other neurodegenerative disorders. Olfactory deficits may be a potential marker for early and differential diagnosis of AD. We aimed to assess olfactory functions in patients with AD and other neurodegenerative disorders, to further evaluate the smell tests using subgroup analysis, and to explore moderating factors affecting olfactory performance.MethodsCross-sectional studies relating to olfactory assessment for both AD and other neurodegenerative disorders published before 27 July 2022 in English, were searched on PubMed, Embase and Cochrane. After literature screening and quality assessment, meta-analyses were conducted using stata14.0 software.ResultsForty-two articles involving 12 smell tests that evaluated 2,569 AD patients were included. It was revealed that smell tests could distinguish AD from mild cognitive impairment (MCI), Lewy body disease (LBD), depression, and vascular dementia (VaD), but not from diseases such as frontotemporal dementia (FTD). Our finding indicated that in discriminating AD from MCI, the University of Pennsylvania Smell Identification Test (UPSIT) was most frequently used (95%CI: −1.12 to −0.89), while the Brief Smell Identification Test (B-SIT), was the most widely used method in AD vs. LBD group. Further subgroup analyses indicated that the methods of smell test used contributed to the heterogeneity in olfactory threshold and discrimination scores in group AD vs. MCI. While the moderating variables including age, MMSE scores, education years in AD vs. LBD, were account for heterogeneity across studies.ConclusionOur finding suggests smell tests have potential value in early differential diagnosis of AD. UPSIT and its simplified variant, B-SIT, are widely used methods in the analyses.Systematic Review Registrationhttps://www.crd.york.ac.uk/PROSPERO/display_record.php? RecordID = 357970 (PROSPERO, registration number CRD42022357970).

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