Objective sensory testing methods reveal a higher prevalence of olfactory loss in COVID-19–positive patients compared to subjective methods: A systematic review and meta-analysis

Abstract

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes coronavirus disease 2019 (COVID-19), has currently infected over 6.5 million people worldwide. In response to the pandemic, numerous studies have tried to identify causes and symptoms of the disease. Emerging evidence supports recently acquired anosmia (complete loss of smell) and hyposmia (partial loss of smell) as symptoms of COVID-19, but studies of olfactory dysfunction show a wide range of prevalence, from 5% to 98%. We undertook a search of Pubmed/Medline and Google Scholar with the keywords “COVID-19,” “smell,” and/or “olfaction.” We included any study that quantified smell loss (anosmia and hyposmia) as a symptom of COVID-19. Studies were grouped and compared based on the type of method used to measure smell loss—subjective measures such as self-reported smell loss versus objective measures using rated stimuli—to determine if prevalence differed by method type. For each study, 95% confidence intervals (CIs) were calculated from point estimates of olfactory disturbances. We identified 34 articles quantifying anosmia as a symptom of COVID-19 (6 objective, 28 subjective), collected from cases identified from January 16 to April 30, 2020. The pooled prevalence estimate of smell loss was 77% when assessed through objective measurements (95% CI of 61.4-89.2%) and 44% with subjective measurements (95% CI of 32.2-57.0%). Objective measures are a more sensitive method to identify smell loss as a result of infection with SARS-CoV-2; the use of subjective measures, while expedient during the early stages of the pandemic, underestimates the true prevalence of smell loss.

Psychophysical Evaluation of the Olfactory Function: European Multicenter Study on 774 COVID-19 Patients

“…For these reasons, most of the studies published so far are based on the analysis of medical records or on patient interviews, often carried out retrospectively and after an important delay regarding the onset of symptoms of patients. These studies can, therefore, underestimate the frequency of olfactory disturbances due to recall-bias or a poor medical history collected at the time of the evaluation, and in any case, they do not allow an effective quantification of the severity of the disturbance [ 31 ]. This point is strengthened regarding the substantial rate of short-term recovery exhibited in the initial multicenter European study identifying the loss of smell as a COVID-19 symptom [ 2 ].…”

Evaluating the Onset, Severity, and Recovery of Changes to Smell and Taste Associated With COVID-19 Infection in a Singaporean Population (the COVOSMIA-19 Trial): Protocol for a Prospective Case-Control Study

“…These findings will enable better identification of asymptomatic or pre-symptomatic carriers of COVID-19 infection, encouraging earlier self-isolation and medical consultation. Currently, a small number of studies have objectively tested smell and taste acuity in patients with COVID-19, with the majority of studies relying on subjective self-report measures [ 29 ]. Given that one-time self-report measures may underestimate the prevalence of olfactory impairment [ 36 , 37 ] and fail to capture temporal changes in sensory acuity, the COVISMIA-19 study provides an objective measurement of smell and taste loss in a sample of patients with COVID-19 that can encourage earlier self-isolation and medical consultation.…”

“…To date, questionnaire measures alone have been published; these studies relied on self-reporting and may not accurately reflect the true extent of smell and taste changes during COVID-19 infection in the absence of a standardized tool for such measurements. Reliance on self-report questionnaire data and variable approaches could help explain the wide variability in the reported prevalence of olfactory dysfunction with COVID-19 infection (5%-98%) [ 29 ]. Therefore, there is a need for objective testing of smell and taste loss in patients infected with COVID-19 using standardized smell and taste stimuli [ 29 - 31 ].…”

“…Reliance on self-report questionnaire data and variable approaches could help explain the wide variability in the reported prevalence of olfactory dysfunction with COVID-19 infection (5%-98%) [ 29 ]. Therefore, there is a need for objective testing of smell and taste loss in patients infected with COVID-19 using standardized smell and taste stimuli [ 29 - 31 ]. Moreover, due to extensive restrictions on movement, the need to socially distance, and the potential for infection spread with reusable standardized odor and taste test materials, the collection of in-person physical data is challenging.…”

NIH Workshop Report: sensory nutrition and disease

“…Several months after the NIH conference was held, the US CDC declared an outbreak of SARS-CoV-2 (i.e., sudden acute respiratory syndrome coronavirus 2), which leads to COVID-19, a pandemic ( 5 ). Abrupt loss of taste and smell is among the most reliable predictors of COVID-19 ( 193 , 194 ), which highlights the urgent need for practical tests of chemosensory loss that are standardized, valid, and reliable ( 195 ). Taste and smell loss with COVID-19 is fully or partially regained, but it is currently unclear what the long-term consequences will be on flavor perception, food preferences, food intake, and broader disease risks.…”