Discussion
In our study, we found that the estimated prevalence of self- reported OD in our sample was 27.4 % while the prevalence of abnormal olfactory test was 16.3% which is lower than what was reported by most recent studies including studies using standard olfactory tests.5 Furthermore, we found that OD was more prevalent in females with confirmed COVID-19 infection which is similar to most published studies.
The low prevalence of OD in our study in comparison with other studies can be explained by the fact that the majority of our patients were hospitalized with severe COVID-19 pneumonia. So, the low prevalence could be explained by the delayed testing which led to the partial recovery of the olfactory dysfunction. While other studies examined patients with mild- moderate COVID-19 disease in their early stage. Moreover, psychophysical tests such as the Q-SIT evaluate one’s sense of smell at a specific point in time, which could lead to an underestimation of the prevalence of OD. Jerome R. et al. showed that about 38.3% of patients with self-reported sudden-onset olfactory dysfunction found to be normosmic by the psychophysical Sniffin’ Sticks test. we recorded similar finding of 30.2% having normal Q-SIT while they subjectively reported OD.17
When we measured the association between the Q-SIT and subjectively reported OD at the time of test, we found better positive predictive value and negative predictive value on a cutoff score ≤ 1 (84% and 93% respectively) in compare to the cutoff score ≤ 2. (Table 5) For that we have used the former cutoff in all previously mentioned calculations.
This study showed no significant association between olfactory dysfunction and nasal symptoms. This is supporting the hypothesis of direct invasion of the olfactory neurons by SARS-CoV-2 as the virus could be replicated in neural cell line U251 in vitro.18 That is against other hypothesis of olfactory cleft blockage due to inflammation or inflammatory cytokines affecting olfactory neural mucosa.6
It was expected to have significant association between OD and abnormal tasting (38.4%) as retronasal olfaction is the cause of most gustatory impairment.19 Moreover, the prevalence of gustatory impairment (31.8%) in our sample was higher than olfactory impairment. These data are supported by similar findings of an epidemiological survey conducted in four European countries.20 while some studies differentiate between olfactory and gustatory dysfunction, others just report the prevalence of both anosmia and Ageusia as one symptom. 7,21,22