Widespread smell testing for COVID-19 has limited application – Authors' reply
Authors: Cristina Menni, Carole H Sudre, Claire J Steves, Sebastien Ourselin, Tim D Spector
Journal: The Lancet (2020)

Since the beginning of the COVID-19 pandemic, there has been much debate worldwide about which of the COVID-19 symptoms should be used for contact tracing to contain the viral spread. In April, 2020, we showed that loss of smell was the strongest single predictor of COVID-19,1Menni C Valdes AM Freidin MB et al.Real-time tracking of self-reported symptoms to predict potential COVID-19.Nat Med. 2020; 26: 1037-1040Crossref PubMed Scopus (920) Google Scholar and anosmia was finally added to Public Health England's COVID-19 case definition on May 18, 2020.2Department of Health and Social CareStatement from the UK Chief Medical Officers on an update to coronavirus symptoms.https://www.gov.uk/government/news/statement-from-the-uk-chief-medical-officers-on-an-update-to-coronavirus-symptoms-18-may-2020Date: May 18, 2020Date accessed: November 2, 2020Google Scholar In our previous Correspondence,3Menni C Sudre CH Steves CJ Ourselin S Spector TD Quantifying additional COVID-19 symptoms will save lives.Lancet. 2020; 395: e107-e108Summary Full Text Full Text PDF PubMed Scopus (66) Google Scholar we also suggested that low-cost so-called smell the difference screening tests could be implemented in some settings to capture a larger number of positives than temperature sensors do. We highlighted the greater potential importance of anosmia, as fever was present only in 42·7% of individuals testing positive, versus anosmia being present in 64·6% (34·7% of those not suffering from fever).2Department of Health and Social CareStatement from the UK Chief Medical Officers on an update to coronavirus symptoms.https://www.gov.uk/government/news/statement-from-the-uk-chief-medical-officers-on-an-update-to-coronavirus-symptoms-18-may-2020Date: May 18, 2020Date accessed: November 2, 2020Google Scholar However, we agree with Claire Hopkins and Barry Smith that recommendations for screening should be more cautious as a high percentage of the population, especially older people, have a pre-existing diminished sense of smell4Brämerson A Johansson L Ek L Nordin S Bende M Prevalence of olfactory dysfunction: the Skövde population-based study.Laryngoscope. 2004; 114: 733-737Crossref PubMed Scopus (466) Google Scholar and should not be discriminated against. There is also the problem of long duration symptoms, increasingly known as long COVID. Using the COVID Symptoms Study app, which now has more than 4 million users,5Sudre C Murray B Varsavsky T et al.Attributes and predictors of long-COVID: analysis of COVID cases and their symptoms collected by the Covid Symptoms Study app.medRxiv. 2020; (published online Oct 21.) (preprint)https://doi.org/10.1101/2020.10.19.20214494Google Scholar we confirm that anosmia is still the single most predictive symptom of a positive swab test across different age groups, with odds ratios ranging from 13·67 (95% CI 11·65–16·02) for the older group to 20·86 (18·62–23·4) for the younger one (appendix). We have also been able to collect data on symptoms duration. Of the 4182 adult app users who (1) were healthy at the time of sign-up, (2) tested positive for severe acute respiratory syndrome coronavirus 2 after registration on the app, with onset of symptoms occurring between 14 days before and 7 days after PCR test, and (3) logged regularly (ie, no gap in reporting of more than 7 days), we found that 54·5% reported anosmia in the first week. In this subset, anosmia appeared on day 2 (IQR 0-5) and had a median duration of 6 (3–11) days, but in 10% of cases, anosmia could persist for 3 weeks or more. In contrast, fever lasted a median of 3 days (1–7), although 10% reported it lasting more than 11 days and only 3·6% for more than 21 days.5Sudre C Murray B Varsavsky T et al.Attributes and predictors of long-COVID: analysis of COVID cases and their symptoms collected by the Covid Symptoms Study app.medRxiv. 2020; (published online Oct 21.) (preprint)https://doi.org/10.1101/2020.10.19.20214494Google Scholar These data suggest that those people with new-onset anosmia should self-isolate and seek testing. However, as anosmia or dysosmia can often be present long after the first 10 days when transmission is less likely,6Centers for Disease Control and PreventionDuration of isolation and precautions for adults with COVID-19.https://www.cdc.gov/coronavirus/2019-ncov/hcp/duration-isolation.htmlDate: Sept 10, 2020Date accessed: September 10, 2020Google Scholar when used as a screening tool, it is crucial to consider the onset of symptoms so as not to discriminate against older people or those with long-term symptoms. We thank all users of the COVID Symptoms Study app. ZOE Global provided in kind support for all aspects of building, running, and supporting the app and service to all users worldwide. The UK Department of Health funds the core data collection and surveys. CM is funded by the Chronic Disease Research Foundation and the Medical Research Council's AimHy project grant. CHS is supported by the Alzheimer's Society. SO is funded by the Wellcome Trust, the EPSRC Centre for Medical Engineering, and Innovate UK (UK Research and Innovation). TDS is consultant for ZOE Global. CJS declares no competing interests. Download .pdf (.17 MB) Help with pdf files Supplementary appendix Widespread smell testing for COVID-19 has limited applicationHaving campaigned to achieve recognition that anosmia (loss of smell) is a highly prevalent symptom of COVID-19,1,2 we were delighted that Public Health England changed the case definition on May 18, 2020.3 We agree with Cristina Menni and colleagues,4 that the added sensitivity attributed to adding anosmia to the case definition (less than 2%) is very likely to be a gross underestimate. Indeed, even the additional 15·9% of cases who are identified when including anosmia5 might still fail to capture the full benefit because access to testing in the UK has been so restricted for patients with mild disease. Full-Text PDF