Olfaction is the chemical sensation of gaseous odorants, and it plays a major role in flavor perception and safety awareness. Olfactory testing is a non-invasive diagnostic procedure used to precisely measure a person’s ability to detect, identify, and distinguish different odors. Measuring this sensory function helps medical professionals understand the extent and likely cause of any decline in a patient’s sense of smell.
Clinical Indications for Olfactory Testing
A physician may order olfactory testing for a variety of medical reasons, most often when a patient reports a decrease or total loss of smell. One of the most common causes of olfactory dysfunction is a post-viral infection. This type of infection can damage the delicate olfactory neuroepithelium in the nasal cavity.
Testing is also standard practice following traumatic brain injury (TBI), where damage to the olfactory nerve fibers can lead to a partial or complete loss of function. Chronic sinonasal issues, such as nasal polyps or severe chronic rhinosinusitis, can create a conductive block that prevents odorants from reaching the sensory neurons. In these cases, the olfactory loss is often due to physical obstruction and inflammation.
Testing is also used as an early diagnostic indicator for neurodegenerative diseases like Parkinson’s disease and Alzheimer’s disease. Olfactory deficits often precede the onset of motor or memory symptoms by years. Testing can help differentiate the underlying cause, as sinonasal conditions typically affect the ability to detect an odor, while neurodegenerative diseases more often impair the ability to identify it.
Common Methods of Smell Assessment
Clinical olfactory assessment utilizes standardized psychophysical tests focusing on three main categories: identification, threshold, and discrimination. Identification tests, such as the University of Pennsylvania Smell Identification Test (UPSIT), measure a person’s ability to correctly name or recognize a specific odor from a list of multiple-choice options. This type of test requires both an intact peripheral olfactory system and higher-level cognitive processing to retrieve the odor’s name from memory.
Threshold tests determine the lowest concentration of an odorant that a person can reliably detect in a forced-choice scenario. These tests often employ pen-like devices containing an odorant diluted across a series of concentrations. During the procedure, the patient is typically presented with a set of three pens—two blanks containing only a solvent and one containing the odorant—and must identify which one has a smell. The test uses a staircase method, where the concentration is progressively lowered and raised to pinpoint the exact detection limit.
Discrimination tests evaluate the ability to distinguish between two different smells. Similar to threshold testing, discrimination is often assessed using pen-like devices and a forced-choice method. The patient is presented with triplets of odorants and must determine which of the three smells is different from the other two. These three test types are often combined into comprehensive batteries to provide a complete profile of a patient’s olfactory function.
Interpreting Olfactory Test Results
The scores from a standardized olfactory test are compared against normative data to arrive at a clinical classification of the patient’s sense of smell. The most common classifications are normosmia, hyposmia, and anosmia. Normosmia indicates a normal sense of smell, with the test score falling within the expected range. Hyposmia is defined as a reduced sense of smell, where the test score is below the normal range but the patient still retains some useful olfactory function.
Anosmia represents the total inability to smell, with a score that indicates no functional capacity to detect or identify odors. In some instances, a patient may be diagnosed with a qualitative disorder. These include parosmia, which is a distorted perception of an odor, or phantosmia, the perception of an odor when none is present.
The resulting classification helps guide the treatment plan by suggesting the likely cause of the dysfunction. For instance, a diagnosis of hyposmia due to sinonasal disease may lead to treatment focused on reducing inflammation or removing polyps. Conversely, if the testing points toward a neurological cause, the patient may be a candidate for olfactory training.