Ground‑Glass Opacity (GGO): Definition, Clinical Interpretation, and Guideline Recommendations

1. Radiologic Definition

GGO is a hazy increase in lung attenuation on CT in which the underlying pulmonary vessels and bronchi remain visible, distinguishing it from consolidation where these structures are obscured (American Thoracic Society/European Respiratory Society). 1

When GGO co‑exists with reticular lines and traction bronchiectasis/bronchiolectasis, it reliably indicates the presence of lung fibrosis (ATS/ERS). 1
In idiopathic pulmonary fibrosis (IPF), isolated GGO most often reflects patchy fibrotic thickening of alveolar septa and intra‑alveolar granulation tissue rather than pure inflammation (ATS/ERS). 1
In interstitial lung diseases other than IPF, isolated GGO usually represents inflammatory cells within the alveolar septum or lumen (alveolitis) (ATS/ERS). 1

Extensive GGO involving more than 30 % of lung parenchyma should raise suspicion for diagnoses other than IPF, such as desquamative interstitial pneumonitis, hypersensitivity pneumonitis, organizing pneumonia, or nonspecific interstitial pneumonia (ATS/ERS). 1

Pattern	Key Imaging Features Involving GGO	Guideline Recommendation
Probable UIP	May show mild GGO with subpleural and basal predominance	Suggests usual interstitial pneumonia (UIP) pattern	[3]
Alternative Diagnosis	Predominant GGO without classic UIP distribution	Points toward non‑UIP entities (e.g., NSIP, HP)	[3]
Acute Exacerbation of Fibrotic Lung Disease	Bilateral GGO, with or without superimposed consolidation, on a background of established fibrosis	Defines acute exacerbation criteria	[5]

Evidence level: not explicitly graded in the cited guideline.

Subsolid nodule – focal GGO with variable solid components, still allowing visualization of internal vessels/bronchi. (Thoracic Surgery Society). 6
Pure (nonsolid) GGO nodule – nodule composed entirely of GGO without any solid portion. (Thoracic Surgery Society). 6
Part‑solid nodule – contains both GGO and solid components. (Thoracic Surgery Society). 6
Pure GGO nodules most often correspond to pre‑invasive or minimally invasive adenocarcinoma and carry an excellent prognosis when appropriately managed (Radiology Society). 9

GGO is a frequent CT pattern in pneumonitis caused by molecular‑targeted agents and immune‑checkpoint inhibitors (Chest Society). 10
Nonspecific interstitial pneumonia (NSIP) pattern – patchy or diffuse GGO with peripheral and lower‑zone predominance. (Chest Society). 10
Diffuse alveolar damage (DAD) pattern – extensive bilateral GGO accompanied by dependent air‑space consolidation. (Chest Society). 10

Pulmonary edema (hydrostatic or permeability) – may manifest as diffuse GGO. (Chest Society). 11
Nonspecific interstitial pneumonia – can present with GGO as a dominant feature. (Chest Society). 10
Hypersensitivity pneumonitis – often shows GGO in a mosaic or centrilobular distribution. (American Thoracic Society). 1
Organizing pneumonia – may produce patchy GGO and consolidation. (Chest Society). 10
Drug‑ or radiation‑induced lung disease – GGO is a typical imaging manifestation. (Chest Society). 10

Dependent atelectasis or poor ventilation of dependent lung zones can mimic pathologic GGO; confirmation with prone inspiratory CT is recommended (ATS/ERS/JRS/ALAT). 3

Note: All strength of evidence statements are based on the cited sources; explicit grading was not provided.