Could a Smell Test Flag Brain Disease Earlier?
- Sreeja Krishnamari
- May 22
- 8 min read
What olfaction reveals about Parkinson's, Alzheimer's, and the prodromal brain years before symptoms arrive.
A patient says food tastes flat. Another notices that coffee smells weaker than it used to, but shrugs it off as aging. A third does not notice any problem at all until formal testing reveals marked impairment across multiple odor categories. For decades, such presentations were treated as peripheral nuisances or, at most, quality-of-life complaints with no diagnostic weight. The newer literature makes that view hard to sustain. Olfactory dysfunction is no longer a footnote in the neurological examination. It is increasingly a signal that something is happening in the brain well before the tremor, the gait change, or the memory slip that typically brings patients through the door.
The shift in thinking is supported by a growing body of prospective and biomarker-linked research. In Parkinson's disease, smell loss can precede motor diagnosis by years and may identify people moving through the earliest stages of synucleinopathy. In Alzheimer's disease, odor identification deficits are now linked to amyloid burden, tau pathology, medial temporal lobe thinning, incident mild cognitive impairment, and later dementia. What makes this especially compelling for clinicians is not that smell loss is disease-specific, because it is not. What matters is that smell testing is cheap, rapid, noninvasive, and biologically close to the brain systems that are most vulnerable earliest.
Parkinson's disease and the pre-motor hyposmia story
The case for olfactory dysfunction as a pre-motor feature of Parkinson's disease rests on both cross-sectional evidence and prospective cohort data accumulated over more than two decades. The foundational observation is straightforward: the vast majority of people with confirmed PD demonstrate significant olfactory impairment, and this deficit does not track with disease duration, stage, or severity of motor signs. It appears to reflect a distinct neuropathological process rather than a downstream consequence of the disease's better-known features.[1]
The prospective evidence is more striking. In a landmark 2004 study by Ponsen and colleagues, 361 asymptomatic first-degree relatives of PD patients were assessed for unexplained hyposmia at baseline.[2] Those who showed both hyposmia and markedly reduced dopamine transporter binding on DAT imaging were at substantially elevated risk: within two years, 10 percent had developed clinical Parkinson's disease. Hyposmic relatives who had not converted still showed faster decline in DAT binding compared with normosmic controls, suggesting that smell loss was indexing a biological process already underway.
A community-based prospective cohort published in 2008 by Ross and colleagues extended this finding beyond high-risk families and into the general population.[3] Among 2,267 older men free of PD and dementia at baseline, those in the lowest quartile of odor identification performance carried an adjusted odds ratio of 5.2 for incident Parkinson's disease within the first four years of follow-up, compared with men in the top two quartiles. This statistic remains one of the most cited in the olfactory biomarker literature, and it has held up across subsequent replication work.
"Men in the lowest quartile of odor identification carried more than five times the adjusted odds of developing Parkinson's disease within four years."
Ross et al., 2008
The biological mechanism underlying these associations is not fully resolved, but neuropathological evidence points to early Lewy body deposition in the olfactory bulb and related structures as a probable driver. Braak staging, which describes the caudal-to-rostral spread of alpha-synuclein pathology in PD, places olfactory and brainstem structures at the earliest stages of pathological involvement, before the substantia nigra becomes affected. This is consistent with olfactory dysfunction appearing before motor symptoms and reinforces the rationale for using smell as a pre-motor screen.
Alzheimer's disease and pathology-linked smell loss
The Alzheimer's disease olfactory literature has developed along a different but equally compelling track. Where the PD literature established prodromal smell loss through prospective incidence studies, the AD literature has increasingly linked olfactory performance to the molecular hallmarks of the disease itself: amyloid-beta deposition, tau accumulation, and structural changes in the entorhinal cortex and hippocampus.
At the population level, a 2018 study by Adams and colleagues found that older U.S. adults with objective olfactory dysfunction had more than twice the odds of developing dementia over the subsequent five years compared with those with normal smell function.[4] This association held after adjustment for a range of demographic and health variables and was robust across different cognitive outcome measures.
More recent work has moved from incidence statistics toward pathological correlation. Tian and colleagues, reporting from a community-based cohort of older adults who underwent both smell testing and PET imaging, found that poorer odor identification predicted incident mild cognitive impairment and was associated with overall and regional amyloid-beta burden at baseline.[5] Crucially, faster olfactory decline over follow-up tracked faster accumulation of both amyloid and tau in olfactory-pathway regions, suggesting that serial smell assessment might offer a window onto disease progression rather than merely cross-sectional risk.
Autosomal-dominant Alzheimer's disease
In a high-specificity cohort of PSEN1 E280A mutation carriers studied by Ramirez-Gomez and colleagues in 2022, worse odor identification correlated significantly with greater cortical amyloid burden, greater entorhinal and inferior temporal tau deposition, worse MMSE scores, and poorer delayed recall, despite participants being non-demented at assessment.[6] This pathology-first evidence is among the strongest available for the claim that olfactory deficits in AD reflect neurodegenerative change, not merely age or confounding health factors.
A 2024 study by Diez and colleagues added a mechanistic dimension to this picture using brain network modeling and olfactory circuit analysis.[7] Odor identification deficits mapped most strongly to tau accumulation in key olfactory-pathway regions, including the entorhinal cortex and adjacent temporal structures. The study's modeling suggested that tau spread moved from medial temporal structures toward the olfactory system rather than the reverse, positioning olfactory dysfunction as a downstream marker of entorhinal tau pathology rather than its origin.
Beyond PD: prodromal synucleinopathies
The relevance of olfactory testing extends beyond idiopathic Parkinson's disease to the broader spectrum of synucleinopathies, including dementia with Lewy bodies and isolated REM sleep behavior disorder. In iRBD, a condition now recognized as one of the most reliable prodromal markers of synucleinopathy, olfactory impairment is highly prevalent and may help stratify phenoconversion risk. People with iRBD who also show significant hyposmia appear to face a faster trajectory toward overt neurological disease than those with preserved olfaction.[8]
This has practical implications for how olfactory testing fits into a prodromal workup. Smell assessment is not expensive, does not require imaging infrastructure, and takes less than fifteen minutes in its full commercial form. When used alongside polysomnography findings, DAT imaging, and emerging alpha-synuclein seed amplification assays from cerebrospinal fluid or peripheral tissues, it can contribute meaningfully to a multimodal risk picture even if it cannot carry diagnostic weight alone.
Key studies at a glance
Key studies at a glance
Olfactory testing in clinical and research settings generally falls into three psychophysical domains: odor detection threshold, odor discrimination, and odor identification. Of these, identification is the most widely studied in the neurodegenerative context, correlates most strongly with pathological markers, and is the easiest to administer and score without specialist equipment. The three tools most commonly referenced in the literature differ primarily in length, format, and normative sophistication.
A 2023 update to UPSIT percentile norms for adults aged 50 and older highlighted a critical practical point: self-reported smell ability agrees only minimally with objective percentile-based classification.[9] People routinely overestimate their olfactory function. This means that asking a patient whether their sense of smell has declined is not a reliable screen. Objective testing, even a brief 12-item B-SIT, captures information that subjective history misses entirely.
What smell cannot tell you
The clinical enthusiasm for olfactory testing must be balanced by an honest account of its limitations. Smell loss is common and nonspecific. It is influenced heavily by age, sex, smoking history, sinonasal disease, head trauma, viral infections including SARS-CoV-2, and a range of medications. A patient who presents with low odor identification scores may be experiencing idiopathic hyposmia as a prodromal neurodegenerative feature, or may have chronic rhinosinusitis, a history of COVID-19, or simply age-related receptor degeneration unrelated to neurological disease.
The sensitivity and specificity of smell tests for PD or AD individually are insufficient to support standalone diagnosis. Fullard and colleagues, reviewing the PD olfactory literature, concluded that olfactory testing works best as a screening instrument within a multimodal framework, flagging individuals who warrant further evaluation rather than confirming disease in its own right.[10] The same logic applies in AD, where olfactory impairment improves risk stratification when combined with cognitive screening and biomarker data but does not replace either.
The multimodal future
The most consequential developments in this space are not happening in olfactory testing in isolation but in its combination with the new generation of fluid and tissue-based biomarkers. Alpha-synuclein seed amplification assays from cerebrospinal fluid now demonstrate high sensitivity and specificity for synucleinopathy in prodromal populations, and research using olfactory mucosal biopsies as an accessible tissue source is advancing steadily.[11] When smell testing is positioned as a first-pass, low-cost filter that identifies who should proceed to more resource-intensive workup, it becomes considerably more clinically useful.
A combined signal of olfactory impairment plus elevated neurofilament light chain has demonstrated a hazard ratio of 2.43 for twelve-year cognitive decline in the Shanghai Aging Study cohort, substantially stronger than either marker alone. This pattern, where smell performance adds independent predictive information to fluid biomarkers, is precisely the kind of evidence that supports integrating olfactory assessment into existing brain-health workflows rather than treating it as a niche specialty interest.
"Smell testing is not ready to replace PET or CSF biomarkers. But as a scalable, inexpensive first filter that routes patients toward appropriate workup, it has no obvious competitor."
Synthesized from Fullard et al. 2017 and De Cleene et al. 2025
What is clinic-ready today
The realistic near-term role for olfactory testing sits in three clinical contexts. First, as a risk-stratification screen in patients presenting with other prodromal features of Parkinson's disease, particularly iRBD, constipation, and unexplained mood or autonomic changes. A patient with iRBD and marked hyposmia on the B-SIT carries a meaningfully different risk profile from one with iRBD and preserved smell. Second, as part of a broader cognitive evaluation in patients presenting with subjective memory complaints or early MCI, where olfactory performance contributes additional information about entorhinal integrity beyond standard neuropsychological testing. Third, in longitudinal monitoring for patients already in disease-modifying trial programs, where serial smell assessment may serve as a low-burden functional outcome measure.
What is not clinic-ready is using smell test scores as a diagnostic criterion for PD or AD in the absence of other supporting evidence. The specificity is too low and the confounders too numerous. The appropriate framing for patients and referring clinicians is that olfactory impairment, identified objectively, raises the prior probability of neurodegenerative disease in the right clinical context and justifies closer follow-up, not that it confirms one.
The Neuraci Perspective
The same olfactory pathway that makes fragrance a gateway to emotion and memory is also one of the brain's earliest alarm systems. Understanding how it fails is inseparable from understanding how it functions. That is the science behind why smell matters at every stage of life.
References & Further Reading
Doty RL, Deems DA, Stellar S. "Olfactory dysfunction in parkinsonism: a general deficit unrelated to neurologic signs, disease stage, or disease duration." Neurology. 1988;38(8):1237-1244. DOI: 10.1212/WNL.38.8.1237. https://pubmed.ncbi.nlm.nih.gov/3399075/
Ponsen MM, Stoffers D, Booij J, et al. "Idiopathic hyposmia as a preclinical sign of Parkinson's disease." Annals of Neurology. 2004;56(2):173-181. DOI: 10.1002/ana.20160. https://pubmed.ncbi.nlm.nih.gov/15293269/
Ross GW, Petrovitch H, Abbott RD, et al. "Association of olfactory dysfunction with risk for future Parkinson's disease." Annals of Neurology. 2008;63(2):167-173. https://pubmed.ncbi.nlm.nih.gov/18232017/
Adams DR, Kern DW, Wroblewski KE, et al. "Olfactory dysfunction predicts subsequent dementia in older U.S. adults." Journal of the American Geriatrics Society. 2018;66(1):140-144. https://pubmed.ncbi.nlm.nih.gov/29164603/
Tian Q, Bilgel M, Moghekar AR, Ferrucci L, Resnick SM. "Olfaction, Cognitive Impairment, and PET Biomarkers in Community-Dwelling Older Adults." Journal of Alzheimer's Disease. 2022;86(3):1275-1285. DOI: 10.3233/JAD-210636. https://pubmed.ncbi.nlm.nih.gov/35147543/
Ramirez-Gomez L, Albers MW, Baena A, et al. "Olfactory Function and Markers of Brain Pathology in Non-Demented Individuals with Autosomal Dominant Alzheimer's Disease." Journal of Alzheimer's Disease. 2022;88(2):721-729. DOI: 10.3233/JAD-220075. https://pubmed.ncbi.nlm.nih.gov/35694937/
Diez I, Ortiz-Teran L, Ng TSC, et al. "Tau propagation in the brain olfactory circuits is associated with smell perception changes in aging." Nature Communications. 2024;15:4809. https://pubmed.ncbi.nlm.nih.gov/38839752/
Pilotto A, et al. "Biofluid Markers and Tissue Biopsies Analyses for the Prodromal and Earliest Phase of Parkinson's Disease." Journal of Parkinson's Disease. 2024;14(s2):S333-S344. DOI: 10.3233/JPD-240007. https://pubmed.ncbi.nlm.nih.gov/38848206/
Brumm MC, et al. "Updated Percentiles for the University of Pennsylvania Smell Identification Test in Adults Aged 50 Years and Older." Chemical Senses. 2023. https://pubmed.ncbi.nlm.nih.gov/37016792/
Fullard ME, Morley JF, Duda JE. "Olfactory Dysfunction as an Early Biomarker in Parkinson's Disease." Journal of Parkinson's Disease. 2017;7(1):43-48. https://pubmed.ncbi.nlm.nih.gov/27567884/
De Cleene N, Schwarzova K, Labrecque S, et al. "Olfactory dysfunction as potential biomarker in neurodegenerative diseases: a narrative review." Frontiers in Neuroscience. 2025;18:1505029. https://pubmed.ncbi.nlm.nih.gov/39840019/
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