Sharp Costophrenic Angles: Normal Finding
Sharp costophrenic angles on a chest X-ray indicate a normal appearance with no significant pleural fluid accumulation or pleural thickening present.
Radiographic Interpretation
Sharp costophrenic angles represent the normal anatomic junction where the diaphragm meets the lateral chest wall. This finding specifically indicates:
- Absence of pleural effusion: The PA chest radiograph becomes abnormal only when approximately 200 mL of pleural fluid accumulates, while lateral chest radiographs can detect as little as 50 mL through posterior costophrenic angle blunting 1
- No pleural thickening: Sharp angles exclude diffuse pleural thickening, which by definition obliterates the costophrenic angle when measuring ≥3 mm thickness 1
- Normal pleural space: The preservation of sharp angles confirms the pleural space is free of pathologic fluid, air, or tissue 1
Clinical Significance
The presence of sharp costophrenic angles effectively rules out several important pathologic conditions:
- Pleural effusion excluded: Obliteration or blunting of the costophrenic angle is the earliest radiographic sign of pleural effusion, so sharp angles indicate no clinically significant fluid collection 1
- No pneumothorax: While pneumothorax in supine patients can cause a "deep sulcus sign" (deepening of the costophrenic angle), sharp angles in upright films indicate normal pleural air content 1, 2
- Absence of empyema: Sharp angles help exclude parapneumonic effusions and empyema, which would cause blunting 1
Quantitative Assessment
Research has established objective criteria for normal costophrenic angles:
- Automated analysis systems demonstrate that sharp angles have specific geometric characteristics that distinguish them from blunted angles with high accuracy (area under ROC curve of 0.83) 3
- The angle measurement provides quantitative information allowing detection of abnormal blunting that may indicate early pleural effusion 3
Important Caveats
Imaging position matters critically: On supine chest radiographs, free pleural fluid layers posteriorly and may not cause typical costophrenic angle blunting, instead appearing as diffuse haziness 1. Therefore, sharp angles on supine films are less reliable for excluding small effusions than on upright PA films.
Detection thresholds vary by modality: While sharp costophrenic angles on PA chest X-ray exclude effusions >200 mL, ultrasound can detect as little as 20 mL of fluid that would not be visible on plain radiography 4. If clinical suspicion for small effusion exists despite sharp angles, ultrasound should be considered.
Subpulmonic effusions can be missed: These collections accumulate beneath the lung base and may preserve apparently sharp costophrenic angles on PA view while showing lateral peaking of the hemidiaphragm 1. Lateral radiographs or ultrasound may be needed if this is suspected clinically.