The most underused diagnostic instrument in medicine sits in the middle of the face.

Why olfaction belongs in your practice

Olfactory dysfunction is one of the earliest detectable signals of several neurodegenerative diseases. Adams et al. (2018) followed nearly three thousand cognitively normal older adults across a five-year window and found that those with olfactory dysfunction at baseline had more than twice the odds of being diagnosed with dementia. Devanand et al. (2015) and several other groups have replicated similar findings for Alzheimer's and Parkinson's disease, with smell loss often preceding clinical diagnosis by years to a decade.

The clinical signal is anatomical. The olfactory bulb is one of the only sensory structures that bypasses the thalamus, projecting directly into the medial temporal lobe and limbic regions where neurodegenerative pathology typically begins. Loss of olfactory acuity is, in many cases, an early read on what is happening in the hippocampus and the entorhinal cortex.

Despite the evidence, olfactory screening is not part of any major adult-care guideline in North America. The tools exist, the validation is robust, and the screen can be administered in five to fifteen minutes. The gap is one of awareness and clinical workflow integration — which is something individual practitioners can address before institutional guidelines catch up.

Three assessment tools worth knowing

Three validated olfactory tests dominate the literature. Each has a different cost, time, and use-case profile. The right tool depends on your setting.

Note: tool selection should reflect your clinical context, patient population, and the specific question you are asking. None of these tools alone is diagnostic; abnormal results indicate the need for further evaluation.

Working with post-COVID smell loss

Post-viral olfactory dysfunction following SARS-CoV-2 infection has been one of the most documented chemosensory disturbances in modern medicine. Estimates of persistent smell loss range from 5 to 15 percent of infected individuals at six months, with some studies showing ongoing impairment at twelve months and beyond.

The clinical encounter is often emotionally complex. Many patients minimise the symptom because they have been told it will recover spontaneously. By the time they present, they have often adapted to the loss in ways that obscure the underlying impact on appetite, mood, intimacy, and a sense of safety in their own home environment. Asking specifically and unhurriedly about smell tends to surface what the patient has stopped mentioning.

The most-supported intervention is structured olfactory training. Sorokowska et al. (2017) meta-analysed the evidence across ages and conditions and concluded that olfactory training reliably improves olfactory function, including in older adults with already-declined olfaction. Subsequent post-COVID-specific work (Hummel and colleagues, Le Bon et al. 2021) has extended these findings to viral-onset smell loss. The protocol is simple: four scents (commonly rose, eucalyptus, lemon, and clove), sniffed deliberately for 15 to 30 seconds each, twice a day, for at least 12 weeks. The active ingredient is attention as much as the scents themselves.

A few practical clinical pearls:

• Document baseline acuity with a brief test (BSIT works well in primary care). Self-report is unreliable; patients often underestimate the degree of loss.
• Distinguish anosmia from parosmia in the history. Parosmia (distorted smell) and phantosmia (smelling things that aren't there) are common in post-COVID recovery and require different counselling. The recovery curve for parosmia is often longer and emotionally harder than for anosmia alone.
• Set realistic recovery expectations. Most studies show meaningful improvement within 12 weeks of consistent training, with continued gains over 6 to 12 months. A small subset of patients have persistent dysfunction beyond two years; these patients benefit from referral to specialist chemosensory clinics where available.
• Address the secondary effects. Loss of smell commonly disrupts appetite (and weight), reduces enjoyment of social meals, and is associated with elevated rates of depression and anxiety. Screening for these is appropriate at any olfactory follow-up.

Selected references

Titles link to Google Scholar.

1. Adams, D. R., Kern, D. W., Wroblewski, K. E., McClintock, M. K., Dale, W., & Pinto, J. M. (2018). Olfactory dysfunction predicts subsequent dementia in older U.S. adults. Journal of the American Geriatrics Society, 66(1), 140–144.
2. Devanand, D. P., Lee, S., Manly, J., Andrews, H., Schupf, N., Doty, R. L., et al. (2015). Olfactory deficits predict cognitive decline and Alzheimer dementia in an urban community. Neurology, 84(2), 182–189.
3. Sorokowska, A., Drechsler, E., Karwowski, M., & Hummel, T. (2017). Effects of olfactory training: a meta-analysis. Rhinology, 55(1), 17–26.
4. Hummel, T., Rissom, K., Reden, J., Hähner, A., Weidenbecher, M., & Hüttenbrink, K. B. (2009). Effects of olfactory training in patients with olfactory loss. Laryngoscope, 119(3), 496–499.
5. Le Bon, S. D., Konopnicki, D., Pisarski, N., Prunier, L., Lechien, J. R., & Horoi, M. (2021). Efficacy and safety of oral corticosteroids and olfactory training in the management of COVID-19-related loss of smell. European Archives of Oto-Rhino-Laryngology, 278(8), 3113–3117.
6. Knight, J. E., Yoneda, T., Lewis, N. A., Muniz-Terrera, G., Bennett, D. A., & Piccinin, A. M. (2023). Transitions between mild cognitive impairment, dementia, and mortality: the importance of olfaction. The Journals of Gerontology: Series A, 78(7), 1284–1291.