Pleural effusion on chest X‑ray
A pleural effusion is an abnormal collection of fluid in the pleural space. It is common, and has many causes: heart failure, pneumonia (parapneumonic effusion and empyema), malignancy, pulmonary embolism, liver and kidney disease, and many others. The chest X-ray is usually how an effusion is first found, but it cannot tell you what the fluid is, and it is insensitive to small volumes. Its appearance also depends heavily on whether the patient was erect or supine. Ultrasound is more sensitive, and BTS guidance recommends it to guide pleural procedures.
1. How to look
- Know the position. Fluid falls. On an erect film it collects at the base; on a supine film it layers behind the lung. See projections.
- Check both costophrenic angles on every film. They must be on the film; if not, say so.
- Follow the hemidiaphragm outline: is it visible, and is its shape normal?
- Look at the mediastinum: is it shifted, and which way?
- Look through the fluid for the cause: consolidation, a mass, a large heart, rib lesions.
2. The erect film: meniscus and volumes
On an erect PA film, fluid first fills the posterior costophrenic recess, then blunts the lateral costophrenic angle. As it increases, it forms a homogeneous opacity at the base with a curved upper edge, the meniscus, which is concave upwards and rises higher laterally than medially. The hemidiaphragm outline is lost beneath it.
Key measurements: how much fluid can you see? [VARIES]
- Erect PA: blunting of the lateral costophrenic angle usually needs roughly 200–300 mL.
- Lateral: fluid in the posterior costophrenic angle may be seen with about 50–75 mL.
- Lateral decubitus (affected side down): as little as about 5–10 mL may layer along the dependent chest wall.
- Ultrasound: detects smaller volumes still, and characterises the fluid (septations, echogenicity).
Figures differ between sources; use them to understand why a normal PA film does not exclude a small effusion. Sources: Radiology Masterclass; Radiopaedia; BTS 2023.
The meniscus shape is created by the way fluid spreads around the lung in three dimensions, rising higher at the edges where the X-ray beam passes through more fluid. A perfectly straight horizontal line instead means air above the fluid: a hydropneumothorax. See pneumothorax.
3. The supine film
On a supine film (ICU, trauma, very unwell patients) the fluid layers along the back of the chest. There is no meniscus. Instead you may see:
- a diffuse hazy veil over the hemithorax, denser towards the base, with lung vessels still visible through it
- loss of the hemidiaphragm outline and a blunted costophrenic angle
- an apical cap, where fluid tracks over the lung apex
- widening of the soft-tissue band along the lateral chest wall
A moderate or even large effusion can be surprisingly hard to see on a supine film, especially if both sides are affected and there is no asymmetry to catch your eye. Ultrasound at the bedside is the practical next step. In trauma, the same appearance may be a haemothorax.
Pearl: veil versus consolidation
A supine effusion is a uniform haze through which vessels can still be seen, without air bronchograms. Consolidation is usually patchier, may have air bronchograms and obscures the vessels within it. Both can coexist.
4. Subpulmonic, loculated and fissural effusions
- Subpulmonic effusion: fluid collects between the lung base and the diaphragm, so the "hemidiaphragm" simply looks high. Clues are a peak displaced laterally (the normal peak is more central), a steep lateral slope, and on the left, an increased distance (more than about 2 cm) between the gastric bubble and the apparent diaphragm.
- Loculated effusion: fluid trapped by adhesions, often in empyema. It may sit against the chest wall as a lens-shaped opacity with obtuse angles, and does not move with position.
- Fissural fluid ("pseudotumour" or "vanishing tumour"): a lens-shaped opacity lying in a fissure, most often the horizontal fissure, typically in heart failure. It resolves with diuresis. See heart failure.
5. The white hemithorax
When one side of the chest is completely or almost completely white, the position of the mediastinum is the key to the cause:
| Mediastinal shift | Likely causes |
|---|---|
| Away from the white side | Massive pleural effusion; large mass; diaphragmatic hernia |
| Towards the white side | Complete lung collapse (for example an obstructing central tumour, mucus plug or a misplaced endotracheal tube); previous pneumonectomy; lung hypoplasia |
| None | Consolidation of the whole lung; mesothelioma or trapped lung; effusion combined with collapse, where the push and pull balance |
A massive effusion without shift away from it should make you think of an underlying collapsed lung, often from a central tumour, or a fixed mediastinum from malignancy. See lobar collapse and lung masses.
6. Empyema, abscess and pleural thickening
A plain film cannot tell a simple effusion from an infected one. Features that raise concern for empyema include loculation, a lens-shaped collection with obtuse angles to the chest wall, and a shape that changes between PA and lateral views. A lung abscess, in contrast, is usually round, makes acute angles with the chest wall, has a thick wall and shows an air–fluid level of similar length on PA and lateral films. On CT, an empyema may show the split pleura sign. Diagnosis rests on pleural fluid sampling, guided by ultrasound.
Pleural thickening and plaques also blunt the costophrenic angle or line the chest wall but do not move or change with position. Calcified pleural plaques from asbestos exposure look like irregular "holly-leaf" shapes when seen face on, often on the diaphragm, and tend to spare the apices and costophrenic angles. Compare with old films: a stable blunted angle for years is likely to be thickening, not fluid.
Clues to the cause on the film
The fluid itself looks the same whatever its cause, but the rest of the film often helps:
- Bilateral effusions with a large heart, upper lobe diversion and septal lines suggest heart failure.
- A unilateral effusion with adjacent consolidation suggests a parapneumonic effusion.
- Nodular pleural thickening, a mass, hilar enlargement or bone destruction raise concern for malignancy.
- Rib fractures after trauma suggest haemothorax.
- A left effusion with pneumomediastinum after vomiting should prompt thoughts of oesophageal rupture (Boerhaave syndrome).
Ultrasound-guided sampling, and often CT, are needed to establish the cause of a new unexplained effusion.
7. Pitfalls and mimics
- A normal erect PA does not exclude an effusion. Up to a few hundred millilitres may be hidden.
- Supine effusions are easily missed or mistaken for consolidation.
- Subpulmonic effusions masquerade as a raised hemidiaphragm.
- Lower-lobe collapse or consolidation also obliterates the diaphragm. Look for volume loss, air bronchograms and a meniscus. Often more than one process is present. See silhouette sign.
- Pleural thickening, old scarring and plaques mimic small effusions; compare with previous films.
- Breast tissue and obesity can make the bases look hazy.
Red flags: act now
Escalate
- A large effusion with respiratory compromise: senior review for ultrasound-guided drainage.
- An effusion with fever and pneumonia that does not settle: possible empyema, which needs sampling and often drainage.
- A white hemithorax with shift towards it or no shift: look for an obstructing lesion or misplaced tube.
- A new unilateral effusion in a patient at risk of cancer: needs investigation (often CT and pleural sampling) through the appropriate pathway.
- An effusion after trauma or a procedure: possible haemothorax.
What to focus on, by role
| Role | Priorities |
|---|---|
| Medical student | Blunted angle and meniscus, approximate detection volumes, and the white-out table. |
| Nurse | Recognise worsening breathlessness and drain problems; understand why ultrasound is used before a procedure. |
| Junior doctor | Supine and subpulmonic effusions, when to request ultrasound, and the importance of finding the cause. |
| Radiographer | Including both costophrenic angles, labelling erect versus supine, and lateral decubitus technique when requested. |
| Advanced / FRCR | Empyema versus abscess, pleural plaques and mesothelioma patterns, and post-pneumonectomy appearances. |
References
- Roberts ME, et al. British Thoracic Society Guideline for pleural disease. Thorax 2023;78(Suppl 3):s1–s42. thorax.bmj.com
- Radiology Masterclass. Pleural effusion. radiologymasterclass.co.uk
- Radiopaedia. Pleural effusion. radiopaedia.org
- Life in the Fast Lane (LITFL). Chest X-ray interpretation. litfl.com
- Royal College of Radiologists. iRefer. irefer.org.uk
For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.