01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Pleural effusion is excess fluid between visceral and parietal pleura. Hydrostatic and oncotic imbalance commonly produces transudative fluid, while pleural inflammation, infection, malignancy, embolic disease and lymphatic obstruction commonly produce exudates. Blood, chyle and pus are specific pleural contents with different pathways. Imaging establishes distribution and guides access; fluid analysis and tissue sampling establish cause.
Radiographic visibility depends on position and volume. Erect fluid collects basally and laterally, producing costophrenic blunting and a concave meniscus. Supine fluid layers posteriorly, giving graded hemithorax opacity without a classic meniscus. Very large effusions may shift the mediastinum away; shift toward opacity suggests associated volume loss. Ultrasound detects smaller collections and shows echogenicity or septation, while contrast-enhanced CT maps pleural thickening, loculation and adjacent lung.
Pleural infection evolves from parapneumonic fluid to complicated biochemical infection and empyema. Sampling must be timely and correctly handled: visible pus establishes empyema; pH in non-purulent fluid guides drainage, with glucose and LDH useful when pH is unreliable or intermediate. Management combines source control and antibiotics; failure to improve prompts reassessment of drain position, residual locules, organism coverage and surgical options.
Key points
- On an erect radiograph, free pleural fluid blunts the costophrenic angle and forms a meniscus; on a supine film it may layer posteriorly as diffuse hemithorax haze.
- Thoracic ultrasound distinguishes fluid from consolidated lung, identifies septations and a safe intervention site, but sonographic complexity alone does not define every cause.
- Suspected pleural infection needs prompt antibiotics and fluid assessment; pus or non-purulent fluid with pH at or below 7.2 supports drainage when a safe ultrasound-accessible volume exists.
- Classify pleural fluid with clinical context and paired serum tests; an exudate narrows mechanism but does not by itself identify infection or cancer.
- Use image guidance for pleural aspiration and drain insertion, document indication and stop if pain, cough or deterioration suggests a complication.
- A negative cytology sample does not exclude pleural malignancy; persistent suspicion requires CT review and an appropriate biopsy strategy.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Dependent homogeneous opacity, costophrenic blunting and a meniscus support free fluid; lateral decubitus imaging can show mobility when it answers a practical question.
A lentiform, non-dependent or fissural opacity plus septated ultrasound fluid suggests loculation. CT can map multiple pockets before drainage planning.
Fever, inflammatory illness, adjacent consolidation and complex fluid raise infection; pleural pH, glucose, LDH, Gram stain and culture determine risk more directly.
Nodular or circumferential pleural thickening, mediastinal pleural involvement, a mass or recurrent unilateral fluid is suspicious, but imaging and cytology can both miss malignancy.
Trauma, procedure, anticoagulation or milky fluid changes sampling and management. CT attenuation and pleural-fluid haematocrit or triglyceride analysis support classification.
New pleuritic pain, oxygen decline, haemodynamic change or radiographic air after intervention requires prompt assessment for pneumothorax, bleeding or organ injury.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Thoracic ultrasoundFirst step - Why
- Confirm fluid, assess accessibility and guide aspiration or drain placement.
- Interpretation and limitations
- Record depth, diaphragm, lung, septations and safe window; lack of a safe pocket may require CT planning or specialist review rather than blind needling.
- 02
Pleural fluid pH - Why
- Identify high-risk complicated pleural infection in non-purulent fluid.
- Interpretation and limitations
- In suspected infection, pH at or below 7.2 supports intercostal drainage when fluid is safely accessible; contamination by local anaesthetic or air and analysis delay can distort the value.
- 03
Pleural and serum protein and LDH - Why
- Classify fluid as transudative or exudative with paired samples.
- Interpretation and limitations
- Light criteria are sensitive for exudates but diuresis can reclassify cardiac fluid; classification begins, rather than completes, the diagnostic reasoning.
- 04
Pleural microbiology and cytology - Why
- Identify infection or malignant cells and direct further testing.
- Interpretation and limitations
- Send infection samples in sterile containers and blood-culture bottles as appropriate; negative culture or cytology does not exclude the suspected process.
- 05
Contrast-enhanced chest CT - Why
- Map pleura, lung, mediastinum and drainage anatomy when infection or malignancy is suspected.
- Interpretation and limitations
- Pleural enhancement, nodularity and loculation refine probability and procedure planning, but histology may be needed for a definitive malignant diagnosis.
04Clinical next stepsHow the result changes management or prompts escalation.
01Worked case: pleural sepsisParapneumonic effusion with persistent feverFirst stepAn adult with pneumonia remains febrile and ultrasound shows a drainable pleural collection.+
- 1Assess sepsis and respiratory status, start or optimise antibiotics, review ultrasound and obtain image-guided pleural fluid without delaying necessary treatment.
- 2Inspect the sample for pus and measure pH promptly in a blood-gas syringe while also sending microbiology, protein, LDH, glucose and other targeted tests.
- 3Insert an appropriately placed intercostal drain when pus, pH at or below 7.2 or other strong high-risk evidence supports source control and access is safe.
- 4Verify improvement clinically and on targeted imaging; if sepsis or collection persists, check tube patency and position, residual locules and early surgical options.
02Unilateral effusionInvestigate an unexplained new collectionA stable adult has a unilateral effusion without an established systemic cause.+
- 1Review symptoms, cancer and asbestos risk, infection, embolic risk, drugs and cardiac, renal or hepatic disease alongside prior imaging.
- 2Use ultrasound-guided aspiration for targeted biochemistry, microbiology and cytology and obtain contrast CT when pleural malignancy or complex anatomy is possible.
- 3EscalationEscalate persistent suspicion after negative cytology to specialist pleural review and an image-guided or thoracoscopic biopsy strategy.
03Therapeutic procedureDrain symptomatic fluid safelyA large effusion contributes to breathlessness and therapeutic aspiration or drainage is planned.+
- 1Confirm the purpose, consent, coagulation context and ultrasound window and use an appropriately trained operator and sterile technique.
- 2Drain in a controlled manner while monitoring pain, cough, dyspnoea, oxygenation and haemodynamics and stop if concerning symptoms occur.
- 3Document volume, appearance, samples, complications and the plan for repeat imaging, drain care and diagnosis.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- In pleural infection, follow temperature, inflammatory markers, oxygen need, drain output and ultrasound or CT evidence of residual collection.
- Inspect a chest drain for secure connections, swinging, bubbling, blockage, displacement and surrounding infection under the local pleural protocol.
- After aspiration, reassess symptoms and observations; obtain radiography when complication is suspected or when the procedure circumstances make it necessary.
- Track every unexplained exudate or recurrent unilateral effusion until a cause, surveillance plan or tissue diagnosis is established.
- Coordinate result ownership for cytology and microbiology because late positive findings can alter the pathway after discharge.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Position changes the sign
Supine pleural fluid often produces diffuse posterior haze instead of a meniscus, so an apparently modest portable opacity can conceal a large volume.
Ultrasound guides more than diagnosis
The examination identifies diaphragm, lung and a safe fluid pocket, converting anatomical recognition into procedural risk control.
Biochemistry needs handling discipline
Pleural pH is clinically useful only when collected and analysed correctly; contamination and delay can produce a misleading drainage decision.
An exudate is a branch point
Light criteria indicate pleural inflammation or permeability but do not distinguish infection, malignancy, embolism and other exudative causes.
Negative cytology has boundaries
Some pleural tumours shed few diagnostic cells, so persistent imaging or clinical suspicion calls for tissue strategy rather than reassurance.
07Common pitfallsFrequent interpretation and management errors.
- 01
Performing a pleural procedure from surface landmarks when real-time or recent thoracic ultrasound guidance is available.
- 02
Using an unvalidated delayed or contaminated pH result as the sole reason to insert or withhold a drain.
- 03
Calling an effusion benign after one negative cytology sample despite nodular pleural thickening or recurrent unilateral fluid.
- 04
Assuming every bilateral effusion is cardiac without checking asymmetry, fever, pain, cancer risk and response to treatment.
- 05
Failing to verify drainage effectiveness when fever, inflammatory markers or loculated fluid persist.