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

• Does not fit into definite, probable or Non-COVID
• Manifests above patterns, but the clinical context is wrong, or suggests an alternative diagnosis (e.g. an interstitial lung disease in a connective tissue disease setting)

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

• Absence of typical features AND
• Multifocal, diffuse, perihilar, or unilateral GGO with or without consolidation lacking a specific distribution and are non-rounded or non-peripheral
• Few very small GGO with a non-rounded and non-peripheral distribution

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

“Imaging features can be seen with (COVID-19) pneumonia, though are nonspecific and can occur with a variety of infectious and noninfectious processes.”