Version July 2025
| AAST grade | Description | Clinical criteria | Imaging criteria (CT findings) | Operative criteria | Pathologic criteria |
| I | Retained hemothorax or pleural effusion with positive pleural fluid cultures | Leukocytosis, fever, chest pain | Pleural fluid consistent with blood or infected fluid; no evidence of loculations | Retained blood, blood clot, or other nonpurulent fluid without lung trapping | Culture of fluid positive for organisms |
| II | Purulent, free-flowing pleural effusion or hemothorax by imaging | Respiratory failure, leukocytosis, fever, chest pain | Pleural fluid consistent with blood or infected fluid; no evidence of loculation | Retained blood, blood clot, with purulent areas or completely purulent fluid | Culture of fluid positive for organisms |
| III | Fibrinopurulent stage with loculated areas of effusion or hemothorax | Respiratory failure, leukocytosis, fever, chest pain | Pleural fluid consistent with blood or infected fluid, with evidence of loculation | Retained blood, blood clot, or other purulent fluid with limited lung trapping | Culture of fluid positive for organisms with areas of fibrous peel |
| IV | Organizing stage with evidence for restricted lung expansion and pulmonary mechanics | Severe respiratory failure, leukocytosis, fever, chest pain | Pleural fluid consistent with blood or infected fluid with evidence of loculation; significantly decreased lung volumes | Retained blood, blood clot, or other purulent fluid with diffuse pleural inflammation and lung trapping | Culture of fluid positive for organisms with areas of fibrous peel |
| V | Spontaneous external drainage or spreading extrathoracic necrotizing soft tissue infection | Severe respiratory failure, leukocytosis, fever, chest pain | Above, plus severe inflammatory changes of adjacent abdominal, mediastinal, or subcutaneous tissue | Above, with external drainage or necrotizing soft tissue infection of mediastinum, diaphragm, or subcutaneous tissues | Above, with necrotic soft tissue from mediastinum, or subcutaneous space |
