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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- Classified as transudate (protein <30 g/L) or exudate (protein >30 g/L) — Light’s criteria if borderline
- Common transudative causes: heart failure, liver cirrhosis, nephrotic syndrome, hypothyroidism
- Common exudative causes: pneumonia (parapneumonic), malignancy, TB, PE, rheumatoid, pancreatitis
- Diagnostic aspiration: pH, protein, LDH, glucose, cytology, MC&S — send pleural and serum samples together
- Empyema (pH <7.2, purulent fluid, or organisms on Gram stain) = urgent chest drain
Overview
A pleural effusion is an accumulation of fluid in the pleural space between the visceral and parietal pleura. Classification is based on Light’s criteria: an effusion is exudative if it meets ANY of: pleural protein/serum protein >0.5, pleural LDH/serum LDH >0.6, or pleural LDH >2/3 the upper limit of normal for serum LDH. Transudative effusions result from increased hydrostatic pressure or decreased oncotic pressure (systemic causes). Exudative effusions result from increased capillary permeability or impaired lymphatic drainage (local causes).
Epidemiology
Pleural effusions are extremely common, affecting approximately 1 million people per year in the UK. Heart failure is the commonest cause of transudative effusions. Malignancy is the commonest cause of exudative effusions in patients over 60 (lung, breast, lymphoma, mesothelioma). Parapneumonic effusions complicate approximately 40% of bacterial pneumonias. TB is an important cause worldwide and in UK populations with higher TB prevalence.
Clinical Features
Symptoms
Progressive breathlessness — worse on exertion, may be positional
Pleuritic chest pain (early/inflammatory) — may resolve as effusion enlarges
Dry cough
Constitutional symptoms (fever, weight loss, night sweats) — suggest malignancy, TB, or empyema
Signs
Stony dull percussion note over the effusion
Reduced or absent breath sounds over the effusion
Reduced vocal resonance and tactile vocal fremitus
Tracheal deviation AWAY from the effusion (if massive)
Signs of underlying cause: raised JVP (heart failure), ascites (liver cirrhosis), cachexia (malignancy)
Investigations
First-line
Chest X-ray (PA erect)Blunting of costophrenic angle (>200 mL needed to detect). Meniscus sign. Large effusions cause mediastinal shift AWAY
Ultrasound-guided diagnostic aspirationSend for: pH, protein, LDH, glucose, cytology, MC&S, and — if TB suspected — adenosine deaminase (ADA)
Paired serum protein and LDHEssential for applying Light’s criteria
Second-line
CT chest with contrastIf malignancy suspected — assess for pleural thickening, nodularity, mediastinal lymphadenopathy, underlying mass
Pleural fluid cytologyPositive in ~60% of malignant effusions on first aspiration; increases to ~75% with repeated sampling
Specialist
Thoracoscopy (medical or VATS)Gold standard for undiagnosed exudative effusion — allows direct pleural biopsy under vision
CT-guided pleural biopsyAlternative to thoracoscopy for pleural thickening or nodularity
Management
BTS Pleural Disease Guidelines 20231
Transudative effusion
- Treat the underlying cause (diuretics for heart failure, albumin for hypoalbuminaemia)
- Therapeutic aspiration only if symptomatic and large
- Do NOT insert a chest drain for uncomplicated transudative effusions
2
Parapneumonic effusion / empyema
- Simple parapneumonic (clear, pH >7.2): antibiotics alone, monitor
- Complicated parapneumonic (pH <7.2, turbid) or empyema (pus, organisms on Gram stain): urgent chest drain + antibiotics
- Intrapleural fibrinolytics (alteplase + DNase) if loculated empyema not draining
- Surgical referral (VATS debridement) if not responding to medical management
3
Malignant effusion
- Therapeutic aspiration for symptom relief (maximum 1.5 L at a time to avoid re-expansion pulmonary oedema)
- Indwelling pleural catheter (IPC) for recurrent symptomatic effusions
- Talc pleurodesis (via chest drain or thoracoscopy) to prevent reaccumulation
- Treat underlying malignancy where possible
Complications
- Empyema: Infected pleural space — requires drainage and prolonged antibiotics
- Trapped lung: Visceral pleural thickening prevents re-expansion
- Re-expansion pulmonary oedema: From too-rapid drainage of large effusion — limit aspiration to 1.5 L
- Respiratory failure: Large effusions compromise ventilation
UKMLA Exam Tips
- 1Light’s criteria: exudate if ANY of — pleural/serum protein >0.5, pleural/serum LDH >0.6, pleural LDH >2/3 upper limit of serum normal
- 2Stony dull percussion = effusion. Resonant = pneumothorax. Dull = consolidation
- 3pH <7.2 in a parapneumonic effusion = complicated/empyema → drain urgently
- 4Bilateral effusions + raised JVP = heart failure (transudate) — treat with diuretics, NOT drainage
- 5Unilateral bloody effusion in a smoker = malignancy until proven otherwise
- 6Mesothelioma: asbestos exposure + pleural thickening/effusion — long latency (20–50 years)
practicetest your knowledge on pleural effusionApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — respiratory and beyond.
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