While bilateral nostril testing detects the majority of clinically meaningful cases of olfactory dysfunction, unilateral testing can detect deficits that are not ordinarily recognized. In principal, bilateral olfactory tests measure the functioning of the better side of the nose. To assess unilateral function, the naris contralateral to the tested side would need to be occluded to prevent or minimize crossing of inhaled or exhaled air at the rear of the nasopharynx to the opposite side (called retronasal stimulation). The patient is instructed to sniff the stimulus normally and to exhale through the mouth.

The majority of patients do not notice this difference between nostrils as long as olfactory function of the better nostril remains in the normal range. Because there is the possibility of unilateral loss of olfactory function or differential degrees of olfactory dysfunction in the two nostrils, unilateral tests of olfactory threshold sensitivity are routinely included in many chemosensory clinical test batteries to supplement bilateral tests. The detection threshold measurement of the smell acuity within each nostril uses the odorant phenyl ethyl alcohol. This test is designed to assess the degree of unilateral olfactory loss.