Unilateral reduced sense of smell is an early indicator for global olfactory loss.

It is estimated that 15% of normosmic subjects demonstrate lateralized smell loss. The majority of individuals will not notice this difference between nostrils as long as olfactory function of the better nostril remains in the normal range. We were interested in the question whether subjects demonstrating clinically significant side differences of olfactory function are at risk to develop bilateral olfactory loss. Normosmic individuals with such side differences ("difference group", n = 35) were re-tested on average 4.6 years after baseline investigations. Additionally, 58 subjects who did not demonstrate side differences ("control group") were also re-investigated. All participants received detailed olfactory testing using the "Sniffin' Sticks" involving tests for odor threshold, odor discrimination, and odor identification. Olfactory testing at follow-up indicated lower olfactory function (p = 0.005) in the "difference group" than in the "control group". The degree of side difference at baseline correlated negatively with the results from olfactory testing at follow-up (r = -0.29; p = 0.01). These results suggest that individuals with side differences of olfactory function are at risk to develop bilateral olfactory loss within 4.5 years. Thus, the degree of lateralized smell function is an indicator for future smell loss.