Pleural effusion

Last updated: 1 September 2026
Reviewed by: Specialist doctors from the Elfcare quality team

Feeling short of breath, noticing a persistent dry cough, or experiencing a heavy feeling in one side of the chest can be easy to dismiss, but these symptoms can sometimes reflect fluid building up around the lungs. The pleural space normally contains only a small amount of fluid to help the lungs move smoothly, and when excess fluid accumulates, it can compress the lung and make breathing increasingly difficult.

Pleural effusions are frequently identified as incidental findings on chest imaging, often before they have caused significant symptoms. Identifying one is clinically important not because the fluid itself is the primary problem, but because it is almost always a signal of an underlying condition that requires investigation and treatment.

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What is a pleural effusion?

Your lungs are wrapped in a thin, two-layered membrane called the pleura. Normally, a tiny amount of fluid sits between these layers so your lungs can expand smoothly when you breathe. A pleural effusion happens when too much fluid builds up in this space, squeezing the lung and making it hard to take a deep breath. Effusions are classified by the nature of the fluid:

  • Passive leakage (transudative): fluid leaks into the lung space due to pressure imbalances from body-wide health issues. It usually affects both lungs and is commonly caused by heart, liver, or kidney failure.

  • Inflammatory buildup (exudative): fluid accumulates because of local damage, inflammation, or blocked drainage around the lung itself. It typically affects just one lung and is tied to pneumonia, infections, or cancer.

The distinction between transudative and exudative effusions is made using Light's criteria applied to pleural fluid analysis, a procedure performed by a specialist. This distinction is the most important clinical question after a pleural effusion is identified, as it directly determines the investigation pathway.

Symptoms of pleural effusion

Small pleural effusions may cause no symptoms and are discovered entirely incidentally on imaging. As fluid accumulates, symptoms reflect progressive lung compression:

  • Shortness of breath on exertion, worsening as the effusion enlarges

  • A dry, non-productive cough from pleural irritation

  • Dull, aching chest heaviness or discomfort on the affected side

  • Reduced breath sounds on the affected side, noticed by a doctor on examination

  • Positional breathlessness, worse when lying flat or on the unaffected side

  • In large effusions: significant respiratory distress and reduced exercise tolerance

Symptoms from the underlying cause frequently accompany the effusion itself, including peripheral oedema and orthopnoea in heart failure, fever and pleuritic chest pain in infection, and unexplained weight loss and fatigue in malignancy.

What causes pleural effusion?

The cause determines the clinical significance and management entirely. Common contributing causes include:

  • Heart failure: the most common cause. Increased pressure in the blood vessels of the lungs allows fluid to accumulate in the pleural space, often affecting both sides.

  • Liver cirrhosis: severe liver scarring allows fluid from the abdomen to travel upward and collect around the right lung.

  • Cancer: tumors like lung or breast cancer irritate the chest lining or block natural fluid drainage.

  • Pneumonia: a nearby lung infection causes intense inflammation, which can turn into trapped, infected fluid.

  • Pulmonary embolism: a blood clot blocking flow to the lungs triggers localized tissue inflammation and fluid buildup.

  • Autoimmune conditions: immune illnesses like lupus or rheumatoid arthritis cause inflammation in the lung lining.

  • Nephrotic syndrome: low blood protein levels and altered fluid balance can contribute to pleural fluid accumulation.

  • Tuberculosis: a specific bacterial infection that causes fluid buildup, especially in high-prevalence areas.

  • Hypothyroidism: severe hypothyroidism is an uncommon but reversible cause of pleural effusion.

How is pleural effusion detected?

Pleural effusion is detected through thoracic imaging, with blood tests identifying the underlying systemic conditions most commonly responsible.

Thoracic MRI Elfcare’s full-body MRI images the thorax and can identify pleural fluid while also providing information about the surrounding lungs, pleura, and mediastinum. This helps characterise the finding and determine whether further assessment is needed.

Blood tests identify the systemic conditions most commonly causing pleural effusions and assess the severity of organ dysfunction. Relevant markers in Elfcare's panel include:

  • NT-proBNP: highly elevated levels strongly point to heart failure as the cause of fluid buildup.

  • Albumin: low levels from liver, kidney, or nutritional issues drop blood pressure controls, causing fluid to leak.

  • Liver function (ALT, AST, GGT, ALP, bilirubin): assess for liver scarring (cirrhosis), which frequently causes fluid to leak into the right lung space.

  • Kidney function (creatinine, cystatin C, eGFR): evaluate for severe kidney damage that triggers body-wide fluid leakage.

  • CRP and full blood count: high levels support an active lung infection, pneumonia, or inflammatory condition.

  • TSH: screens for a severely underactive thyroid, which is a fully reversible cause of fluid accumulation.

Why early detection matters

A pleural effusion is a symptom of an underlying condition, not a diagnosis in itself. Identifying it early, before it has caused significant lung compression or before the underlying condition has progressed, provides the widest window for effective intervention. Heart failure identified as the cause of a bilateral effusion can be optimised with medication before further cardiac remodelling occurs. Malignancy identified as the cause of a unilateral exudative effusion can be investigated and staged at the earliest possible point. Empyema from an infected effusion requires urgent drainage to prevent life-threatening sepsis. An incidentally identified small effusion on Elfcare's thoracic MRI can initiate a diagnostic pathway that fundamentally changes the clinical course of the underlying condition responsible for it.

How Elfcare can help

Elfcare's full body MRI includes the thorax and can identify pleural effusions while providing information about the surrounding lungs and pleural surfaces. This can help determine whether further investigation is needed.

Our blood panel covers NT-proBNP, albumin, liver function, kidney function, CRP, and thyroid function, providing the systemic picture needed to identify the most common underlying conditions driving pleural fluid accumulation.

If our MRI or blood tests identify a pleural effusion or findings consistent with its underlying cause, we take care of further diagnostics or refer you to the appropriate specialist.

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Summary

Pleural effusion is an abnormal accumulation of fluid in the space surrounding the lungs that compresses lung tissue and impairs breathing. It is almost always a sign of an underlying systemic condition, most commonly heart failure, malignancy, infection, liver cirrhosis, or autoimmune disease, rather than a primary diagnosis in itself. Elfcare's full body MRI directly images the pleural spaces and identifies effusions alongside contextual assessment of the surrounding structures, while our blood panel covers the key systemic markers for identifying the underlying cause. Early identification of a pleural effusion and the condition driving it provides the greatest opportunity for targeted treatment that protects long-term respiratory and systemic health.

Last updated: 1 September 2026
Reviewed by: Specialist doctors from the quality team at Elfcare

FAQ

  • A pleural effusion is an abnormal accumulation of fluid in the pleural space, the cavity between the two layers of membrane surrounding each lung. It compresses the adjacent lung and impairs breathing. Effusions are classified as transudative, from systemic pressure or protein imbalances, or exudative, from local inflammation, infection, or malignancy. The distinction determines the investigation pathway and management.

  • Shortness of breath worsening with exertion, a dry non-productive cough, and dull chest heaviness on the affected side. Small effusions are frequently asymptomatic and discovered incidentally on imaging. Large effusions cause significant respiratory distress and markedly reduced exercise tolerance.

  • Heart failure is the most common cause of bilateral transudative effusion. Malignancy, pneumonia, pulmonary embolism, autoimmune conditions, liver cirrhosis, nephrotic syndrome, and tuberculosis are important causes of exudative or transudative effusion depending on the mechanism. Identifying the underlying cause is the most critical clinical step after a pleural effusion is found.

  • Thoracic MRI directly images the pleural spaces, identifying fluid accumulation and assessing adjacent structures for the underlying cause. Blood tests covering NT-proBNP, albumin, liver and kidney function, and inflammatory markers identify the systemic conditions most commonly responsible. Pleural fluid analysis through thoracocentesis, performed by a specialist, provides definitive classification and diagnosis.

  • Yes. Elfcare's full body MRI covers the thorax as standard, directly identifying pleural fluid accumulation and assessing the surrounding structures. Our blood panel covers the key systemic markers for identifying the underlying cause. If a pleural effusion or related finding is identified, we take care of further diagnostics or refer you to the appropriate specialist.

  • Yes. Treatment targets both the effusion itself and its underlying cause. Small asymptomatic effusions are monitored. Symptomatic effusions are drained through thoracocentesis for immediate relief. Recurrent malignant effusions are managed with pleurodesis or an indwelling pleural catheter. Heart failure-related effusions respond to optimised diuretic therapy. Infected effusions require urgent drainage and antibiotics. Treating the underlying cause is the most important intervention for preventing recurrence and protecting long-term respiratory health.