On chest X-ray of a patient with right-sided pleural effusion, the fluid blunts the costophrenic angle but on lateral decubitus view, the fluid shifts freely. This indicates:
- A Exudative effusion — more likely than transudate to be loculated
- B Free (non-loculated) effusion — volume can be estimated as >200 mL if blunting costophrenic angle ✓
- C Subpulmonic effusion — does not shift on decubitus
- D Haemothorax — does not shift freely
Correct answer: B. Free (non-loculated) effusion — volume can be estimated as >200 mL if blunting costophrenic angle
Explanation
Freely shifting fluid on lateral decubitus confirms a non-loculated (free) pleural effusion. Blunting of the posterior costophrenic angle requires approximately 200 mL. Free-shifting fluid does not exclude either exudate or transudate (Light's criteria are needed). Subpulmonic effusion may not shift freely and can mimic an elevated hemidiaphragm. Haemothorax can be free or clotted — acute haemothorax may shift freely initially.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.