The sense of smell, known as olfaction, is a complex chemical process allowing humans to detect and interpret airborne molecules. Clinical testing of olfaction is a standardized procedure used by medical professionals, primarily otolaryngologists and neurologists, to objectively measure a patient’s function. These tests move beyond simple self-reporting to provide a measurable score reflecting the health of the entire olfactory system. The ability to smell is important because it affects quality of life, warns of dangers like gas leaks or spoiled food, and is closely linked to flavor perception.
When and Why Olfactory Testing is Performed
Doctors order olfactory testing when a patient reports a sudden or gradual change in their ability to smell, or as part of a broader diagnostic workup. A frequent trigger is a loss of smell following an upper respiratory tract infection, such as the common cold or a viral illness like COVID-19. Head trauma is another common cause, as a forceful blow can sever the delicate olfactory nerves connecting the nose to the brain.
The test is also used to assess neurological health, as impaired olfaction can be an early sign of neurodegenerative disorders. Conditions like Parkinson’s disease and Alzheimer’s disease often cause a decline in the sense of smell many years before other motor or cognitive symptoms appear. Patients with chronic sinus issues or nasal polyps may undergo testing to determine if a physical blockage is preventing odor molecules from reaching the sensory receptors. Olfactory testing provides a baseline measurement and helps determine the cause and severity of the dysfunction, which guides treatment decisions.
Common Subjective Testing Procedures
Objective olfactory testing relies on standardized, commercially available kits that require the patient to actively participate and report their perceptions. These methods measure three distinct aspects of smell function: threshold, identification, and discrimination. The results from these three components are often combined into a single total score, such as the Threshold-Discrimination-Identification (TDI) score in the Sniffin’ Sticks test.
Threshold Tests
Threshold testing determines the lowest concentration of a specific odorant a person can reliably detect. This procedure uses a series of pens or vials containing the same odorant, such as n-butanol or phenylethyl alcohol, at progressively weaker dilutions. A common approach is the forced-choice staircase method, where the patient is presented with three pens: one containing the diluted odorant and two containing only the odorless solvent. The patient must identify which pen has the smell. The test is repeated, with the concentration increasing or decreasing based on the patient’s responses, until the minimum detectable concentration is pinpointed.
Identification Tests
Identification tests assess the ability to recognize and name a smell, requiring both sensory detection and cognitive processing. The University of Pennsylvania Smell Identification Test (UPSIT) is a widely used example, consisting of 40 scratch-and-sniff strips embedded in four booklets. After scratching the strip, the patient smells the released odor and chooses the correct name from a list of four options. Because it is a forced-choice format, patients must select an answer even if they do not perceive an odor, which helps distinguish a true deficit from a lack of effort. Standardized kits like the Sniffin’ Sticks also include identification subtests, often using common scents like coffee, leather, or peppermint.
Discrimination Tests
Discrimination tests measure the ability to tell the difference between two different odors. This component focuses on the fine-tuning of the olfactory system, independent of the ability to name the scent. The patient is typically presented with three odor-impregnated pens in a triplet set. Two of the pens contain the same odor, while the third contains a different smell. The patient’s task is to identify the single pen that smells different from the other two.
Understanding the Test Results
The total score from a comprehensive olfactory test is compared against large normative databases, which provide expected scores based on age and gender. This comparison allows the clinician to classify the patient’s function accurately, as olfactory performance naturally declines with age and women generally score higher than men. Scores are typically divided into categories of normal function, reduced function, or total loss.
The test results help diagnose two main categories of olfactory impairment: quantitative and qualitative disorders. Quantitative disorders describe a measurable reduction in smell perception. The most severe is anosmia (complete inability to detect odors), while hyposmia refers to a significantly reduced sense of smell.
Qualitative disorders describe a distortion or hallucination of smell perception. Parosmia is a qualitative disorder where familiar smells are perceived as distorted, often becoming foul or unpleasant. Phantosmia is the sensation of smelling something that is not physically present, which is an olfactory hallucination. The objective scores help the medical team determine the severity and type of impairment to develop an appropriate management plan.