The patient is asked to close the eyes and one nostril as the examiner brings a test substance close to the patient's other nostril. The patient is instructed to sniff the test substance. The substance must be volatile and nonirritating, such as cloves, vanilla beans, freshly ground coffee, or lavender. The use of an irritating agent such as alcohol would involve cranial nerve V, as well as cranial nerve I, and the test results would be inaccurate.
Each nostril is tested separately. The examiner asks the patient to identify the test material. A unilateral loss of smell, known as unilateral anosmia, is more important than a bilateral loss, because it indicates a lesion affecting the olfactory nerve or tract on that side.
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