Between March 21, 2020 and April 11, 2020, 763 patients underwent RT-PCR and non-contrast low dose chest CT scan. In 182 patients, RT-PCR was positive for COVID-19 and CT was performed before or within a time interval of four days of the RT-PCR; patients with a positive RT-PCR in a time interval of more than four days after the index CT were excluded because of the possible later onset of the infectious disease. All CT’s were evaluated for the presence of CT-abnormalities and their frequency, to determine typical and atypical findings of COVID-19 pneumonia.
Regarding the patient outcome, each patient was categorized in the highest achieved stage of 5 progressive stages (quarantine at home, admission to a non-ICU, admission to the intensive care unit, intubation at the intensive care unit and mortality) and the duration of hospitalization was determined.
Chest CT scan parameters
All CT examinations were performed on a 128 detector-row CT scanner (Siemens Definition Flash, Forchheim, Germany) with a single breath hold. A non-contrast low dose protocol was performed with the following parameters (gantry speed of 0.5 s per rotation, slice collimation: 128 x 0.6 mm, pitch factor 1.2, slice thickness 1 mm & 3 mm, slice increment 0.7 mm & 3 mm), except for mAs and kV settings depending on patient weight (<50 kg: 80 kV and 30 mAs; 50-80 kg: 120 kV and 20 mAs; >80 kg: 140 kV and 28 mAs).
Evaluation of CT findings and severity
Chest CT scans were evaluated for CT findings by four experienced thoracic radiologists. Typical CT findings described for COVID-19 include ground-glass opacities (GGO), consolidations, crazy-paving pattern, subpleural reticulation, air bronchogram, (reversed) halo sign, subpleural bands, vascular dilatation, focal pleural thickening and airway changes (1). Examples of atypical CT findings are centrilobular nodules, tree-in-bud pattern, enlarged lymph nodes, pleural effusion and cavitation (1). Based on the CT examination, patients were categorised into 3 groups: consistent, inconclusive and inconsistent for COVID-19 pneumonia. The consistent group was very suggestive for COVID-19 because of the presence of typical CT findings for COVID-19 pneumonia. In the inconclusive group, patients had typical findings as well as atypical CT findings and/or co-findings (e.g. signs of heart failure, tumoral masses, coinfection, hypoventilation GGO,...) that make it more difficult to exclude an underlying COVID-19 pneumonia. In the inconsistent group, CT was either normal or compatible with non-COVID pathology.
For estimating the severity, a visual scoring of the lung injury per lobe in 5 categories was used (0: no involvement, 1: 0-5% involvement, 2: 5-25% involvement, 3: 25-50% involvement, 4: 50-75% involvement, 5: >75% involvement).
Ordinal logistic regression (univariate and multivariate) analysis were performed to find which CT findings of COVID-19 positive patients are predictive for patient outcome and linear regression analysis was performed to assess prognostic factors for the hospitalization duration. Statistical analysis was performed using the IBM Statistical Package for Social Sciences (SPSS version 13, IBM Corp., Armonk, NY, USA); statistical significance level was 0.05.