Prevalence of Olfactory Dysfunction Among COVID-19 Patients with Self-Reported Smell Loss Versus Objective Olfactory Tests: A Systematic Review and Meta-Analysis
Background: Olfactory dysfunction (OD) in COVID-19 presents as a sudden onset smell loss commonly seen in mild symptomatic cases with or without rhinitis but can occur as an isolated symptom. The reported prevalence of OD among COVID-19 patients ranged from 5% to 98%. Although numerous studies have been conducted about their association, these were mainly based on self-reported cases and subjective questionnaires.
Objective: This study investigates whether there is a significant difference in the prevalence of olfactory dysfunction between self-reported and objective testing using validated objective olfactory tests among RT-PCR confirmed COVID-19 patients.
Methods: PubMed (MEDLINE), Cochrane, Web of Science, and Google Scholar were searched for studies investigating the prevalence of OD by using objective olfactory tests among patients who self-reported OD (November 1, 2019 to July 31, 2020). All studies were assessed for quality and bias using the Cochrane bias tool. Patient demographics, type of objective olfactory test, and results of self-reported OD and objective testing were reported.
Results: Nine studies encompassing 673 patients met the inclusion criteria. Validated objective olfactory tests used in the included studies were CCCRC, SST and SIT. Overall prevalence of OD among patients who self-reported was higher after objective testing (71% versus 81%). This was also seen in when we performed subgroup analysis based on the objective tests that were used. However, meta-analysis using random effects model showed no significant difference in the overall prevalence of OD (p value=.479, 95% CI 56.6 to 84.0 versus 71.2 to 89.8) as well as in the subgroups.
Conclusion: To the best of our knowledge, this is the first meta-analysis that statistically reviewed articles that evaluated the difference between self-reported and objective tests done on the same patients. Results showing that self-reporting OD approximates the results of the objective tests among COVID-19 positive patients may imply that self-reporting can be sufficient in contact tracing and triggering swabbing and self quarantine during the time of COVID-19 and objective tests can be used as an adjunct in the diagnosis particularly in research. However, this study was limited by small sample size and articles done in European countries hence, interpretation and application of the results of this study must be approached with care. Further studies documenting the difference between self-reporting and objective test in large scale setting involving different countries may be helpful in establishing a definitive consensus.
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