Ruling out COVID-19 by chest CT at emergency admission when prevalence is low: the prospective, observational SCOUT study

Abstract

Background: It is essential to avoid admission of patients with undetected corona virus disease 2019 (COVID-19) to hospitals' general wards. Even repeated negative reverse transcription polymerase chain reaction (RT-PCR) results do not rule-out COVID-19 with certainty. The study aimed to evaluate a rule-out strategy for COVID-19 using chest computed tomography (CT) in adults being admitted to the emergency department and suspected of COVID-19.

Methods: In this prospective, single centre, diagnostic accuracy cohort study, consecutive adults (≥ 18 years) presenting with symptoms consistent with COVID-19 or previous contact to infected individuals, admitted to the emergency department and supposed to be referred to general ward were included in March and April 2020. All participants underwent low-dose chest CT. RT-PCR- and specific antibody tests were used as reference standard. Main outcome measures were sensitivity and specificity of chest CT. Predictive values were calculated based on the theorem of Bayes using Fagan's nomogram.

Results: Of 165 participants (56.4% male, 71 ± 16 years) included in the study, the diagnosis of COVID-19 was confirmed with RT-PCR and AB tests in 13 participants (prevalence 7.9%). Sensitivity and specificity of chest CT were 84.6% (95% confidence interval [CI], 54.6-98.1) and 94.7% (95% CI, 89.9-97.7), respectively. Positive and negative likelihood ratio of chest CT were 16.1 (95% CI, 7.9-32.8) and 0.16 (95% CI, 0.05-0.58) and positive and negative predictive value were 57.9% (95% CI, 40.3-73.7) and 98.6% (95% CI, 95.3-99.6), respectively.

Conclusion: At a low prevalence of COVID-19, chest CT could be used as a complement to repeated RT-PCR testing for early COVID-19 exclusion in adults with suspected infection before referral to hospital's general wards. Trial registration ClinicalTrials.gov: NCT04357938 April 22, 2020.

Keywords: COVID-19; Computed tomography; Prevalence; Reverse transcriptase polymerase chain reaction; Sensitivity and specificity; Severe acute respiratory syndrome coronavirus 2.

Fig. 1

In-hospital patient flow. COVID-19, coronavirus disease 2019; CT, low-dose chest computed tomography; RT-PCR, reverse transcription polymerase chain reaction

Fig. 2

Flow of participants through the study. AB, antibody test; COVID-19, coronavirus disease 2019; CT, low-dose chest computed tomography; RT-PCR, reverse transcription polymerase chain reaction

Fig. 3

Diagnostic characteristics of low-dose chest CT and impact on participant admission to isolation ward. a Doughnut chart represents proportions and b contingency tables frequencies of true positive, false negative, true negative, and false negative diagnosed participants. Chest CT diagnostic performance is given for both the reference standard of RT-PCR test only and the reference standard of RT-PCR and/or AB test. Test characteristics are presented with 95% confidence intervals. AB, antibody test; COVID-19, coronavirus disease 2019; CT, low-dose chest computed tomography; LR + , positive likelihood ratio; LR-, negative likelihood ratio; RT-PCR, reverse transcription polymerase chain reaction

Fig. 4

Fagan`s nomogram showing probability of COVID-19 infection after chest CT diagnosis. a Probabilities were calculated based on the reference standard of RT-PCR test only or b RT-PCR and/or AB test. Positive CT diagnosis (blue arrow) refers to typical or nonspecific appearance, and negative CT diagnosis (red arrow) to atypical or negative appearance of CT scan. Precision is given as 95% confidence interval. Disease prevalence is derived from the number of COVID-19 positive and negative participants. AB, antibody test; CT, low-dose chest computed tomography; LR+, positive likelihood ratio; LR−, negative likelihood ratio; NPV, negative predictive value; PPV, positive predictive value; RT-PCR, reverse transcription polymerase chain reaction