Diagnostic performance of chest computed tomography during the epidemic wave of COVID-19 varied as a function of time since the beginning of the confinement in France

Abstract

Objective: To evaluate the diagnostic performance of the initial chest CT to diagnose COVID-19 related pneumonia in a French population of patients with respiratory symptoms according to the time from the onset of country-wide confinement to better understand what could be the role of the chest CT in the different phases of the epidemic.

Material and method: Initial chest CT of 1064 patients with respiratory symptoms suspect of COVID-19 referred between March 18th, and May 12th 2020, were read according to a standardized procedure. The results of chest CTs were compared to the results of the RT-PCR.

Results: 546 (51%) patients were found to be positive for SARS-CoV2 at RT-PCR. The highest rate of positive RT-PCR was during the second week of confinement reaching 71.9%. After six weeks of confinement, the positive RT-PCR rate dropped significantly to 10.5% (p<0.001) and even 2.2% during the two last weeks. Overall, CT revealed patterns suggestive of COVID-19 in 603 patients (57%), whereas an alternative diagnosis was found in 246 patients (23%). CT was considered normal in 215 patients (20%) and inconclusive in 1 patient. The overall sensitivity of CT was 88%, specificity 76%, PPV 79%, and NPV 85%. At week-2, the same figures were 89%, 69%, 88% and 71% respectively and 60%, 84%, 30% and 95% respectively at week-6. At the end of confinement when the rate of positive PCR became extremely low the sensitivity, specificity, PPV and NPV of CT were 50%, 82%, 6% and 99% respectively.

Conclusion: At the peak of the epidemic, chest CT had sufficiently high sensitivity and PPV to serve as a first-line positive diagnostic tool but at the end of the epidemic wave CT is more useful to exclude COVID-19 pneumonia.

Fig 1. Study flow chart.

Fig 2. R0, number of COVID 19 patients admitted in the hospital and number of positive RT-PCR in our institution.

Number of COVID-19 patients admitted in the hospital in the whole area around Paris (Ile de France: IDF) and in the city of Paris (Paris) (left axis) and number of positive RT-PCR in our institution (PSL) (right axis) and value of R0 as a function of weeks elapsed since the beginning of the confinement W1 = 18–24 of March, W2 = 25–31 of March, W3 = 01–07 of April, W4 = 08–14 of April, W5 = 15–21 of April, W6 = 22–28 of April, W7 = 29 of April-5 of May, W8 = 06–12 of May.

Fig 3. Lung CT patterns found in patients with and without COVID-19 pneumonia.

A) Unenhanced chest CT images of a 62-year-old man with fever and dyspnea revealing one of the typical CT patterns for COVID-19 pneumonia. Axial images show ground glass opacities, crazy paving pattern (arrows) and consolidation with air bronchogramm (arrowhead) mostly distributed in subpleural regions and involving all lobes. RT-PCR here was positive (suggesting true positive diagnosis of CT). B) Unenhanced chest CT images of a 56-year-old man with COVID-19 pneumonia and sarcoidosis. Axial CT images revealed bilateral ground glass opacities (arrows) and perilymphatic irregular nodular thickening in an upper/mid lung distribution (arrowheads). RT PCR was positive (suggesting true positive diagnosis of CT). C) Enhanced chest CT images of a 37-year-old woman with dyspnea and fever revealing Pneumocystis Jiroveci infection. Axial CT images shows central diffuse GGO bilateral falsely considered suggesting of COVID-19. RT-PCR was here negative (suggesting false positive diagnosis of CT). D) Unenhanced chest CT images of a 67-year-old woman with dyspnea for few days revealing a hypersensitivity pneumonitis. Axial CT images shows homogeneous GGO bilateral and symmetric with a bronchovascular distribution. CT was considered suggesting another diagnosis than COVID-19. RT-PCR was here negative (suggesting true negative diagnosis of CT).