The British Society of Thoracic Imaging (BSTI) guidelines (attached below) have outlined features that define Non COVID:

• Lobar pneumonia
• Cavitating infections
• Tree-in bud/ centrilobular nodularity
• Lymphadenopathy, effusions
• Established pulmonary fibrosis

Radiological Society of North America (RSNA) guidelines (attached below) have outlined features that define Atypical COVID:

• Absence of typical or indeterminate features AND presence of:
• Isolated lobar or segmental consolidation without GGO
• Discrete small nodules (centrilobular, “tree- in-bud”)
• Lung cavitation
• Smooth interlobular septal thickening with pleural effusion

The RSNA have also provided some guidance on structuring CT reports for Indeterminate COVID:

“Imaging features are atypical or uncommonly reported for (COVID-19) pneumonia. Alternative diagnoses should be considered.”