Pleural effusion on the chest X-ray
- Finding
Fluid around the lung sinks to the bottom of the chest and shows as a white area with a curved upper edge. Its look changes with the patient's position.
What it is
A pleural effusion is fluid between the two layers of pleura. In an upright patient it falls to the lowest part of the chest: first the posterior costophrenic angle (seen on the lateral view), then the lateral angles on the PA film. It forms a homogeneous white opacity with a concave upper edge that climbs higher at the side wall (the meniscus), and it hides the hemidiaphragm. Lying down, the same fluid spreads behind the lung and only makes the whole hemithorax hazier. A large effusion pushes the mediastinum away. Ultrasound finds smaller amounts than the X-ray, shows whether the fluid has septa, and guides the needle.
What it looks like
- Upright: blunted costophrenic angle and a homogeneous opacity with a concave meniscus, higher laterally.
- The lateral view shows a small effusion earlier (posterior angle) than the PA view; ultrasound smaller still.
- Supine: a veil-like haze over the hemithorax, vessels still visible through it, sometimes an apical cap; no meniscus.
- Subpulmonic effusion: an apparently raised hemidiaphragm whose peak is moved laterally; on the left, a wide gap between lung base and stomach gas.
- Large effusion: white-out with the mediastinum pushed to the other side.
- Loculated or fissural fluid does not move with position; fluid in a fissure can look like a mass that vanishes with treatment.
Why it looks that way
- Free fluid obeys gravity, so its shape depends on whether the film is taken upright, supine or lying on one side.
- The meniscus is the fluid rising higher along the chest wall, where the lung recoils least.
- Fluid has the same density as the diaphragm, so the diaphragm outline is lost (silhouette sign).
Differential and pitfalls
- White hemithorax with shift away: large effusion. Shift toward: collapse of the whole lung. No shift: both, or a mediastinum fixed by tumour.
- Bilateral effusions with a big heart: heart failure first; a unilateral or disproportionate one needs a tap.
- A supine trauma film can hide a large haemothorax; look for a difference in haze between the two sides.
- A raised hemidiaphragm may be a subpulmonic effusion; ultrasound or a decubitus view settles it.
- Pleural thickening and masses can mimic fluid; ultrasound separates them.
Next step
- Point-of-care or formal ultrasound to confirm fluid, look for septa and mark the site for a tap.
- New unilateral effusion without an obvious cause: diagnostic thoracentesis under ultrasound and Light's criteria on the fluid.
- Known heart failure with bilateral effusions: treat the heart failure and recheck; tap if it fails to resolve, is one-sided, febrile or painful.
- Suspected empyema, loculation or malignancy: contrast CT of the chest and early contact with the chest team.
- Breathless with a large effusion or mediastinal shift: urgent senior review for therapeutic drainage.
Test yourself
Think of the answer, then tap the question to see it.
What is the meniscus sign of a pleural effusion on an upright film?
A concave upper edge of the opacity that rises higher along the lateral chest wall than it is medially.
How does a large effusion look on a supine film?
As a diffuse haze over the whole hemithorax with vessels still visible through it and no meniscus, because the fluid layers behind the lung.
A hemithorax is completely white and the trachea is pulled toward it. Effusion or collapse?
Collapse of the whole lung: volume loss pulls the mediastinum toward the opacity, whereas a large effusion pushes it away.
Which study confirms a small effusion and guides the needle?
Ultrasound: it detects smaller amounts than the X-ray, shows septa and marks a safe site for thoracentesis.
Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Pleural Effusion (StatPearls (NCBI Bookshelf), 2024); Chest and Mediastinal Imaging (StatPearls (NCBI Bookshelf), 2026).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.