Unilateral and bilateral pleural effusion

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Pleural effusion is a sign, not a diagnosis. Transudates reflect systemic hydrostatic/oncotic imbalance; exudates reflect pleural inflammation, infection, malignancy, vascular disease or lymphatic disruption.

Laterality changes pre-test probability but is not absolute. Heart failure can be unilateral, PE can be bilateral and malignancy can coexist with cardiac failure; discordant features deserve investigation.

The safest pathway combines history, radiology and ultrasound before sampling, then interprets fluid alongside paired serum results and the clinical question rather than treating a single biochemical cut-off as the diagnosis.

Pleural intervention is a procedure with bleeding, organ injury, pneumothorax and re-expansion risks. Consent, ultrasound, asepsis, positioning, monitoring and a written drainage limit are essential.

Key points

Confirm pleural fluid with chest radiography and thoracic ultrasound; ultrasound must guide the site of any pleural aspiration or drain insertion.
Bilateral, symmetric effusions with a classic heart, renal, liver or hypoalbuminaemic picture can often be treated for the systemic cause first; sample when features are atypical, unilateral/asymmetric, febrile, painful or non-responsive.
An unexplained unilateral effusion usually needs ultrasound-guided aspiration plus blood tests and contrast CT selected around the malignancy/infection question.
Send pleural fluid for protein and LDH with paired serum samples, cytology, microbiology, glucose and pH as clinically indicated; inoculate microbiology blood-culture bottles when infection is suspected.
Light's criteria define an exudate when any one is present: fluid/serum protein ratio above 0.5, fluid/serum LDH ratio above 0.6, or fluid LDH above two-thirds the serum laboratory upper limit.
In suspected pleural infection, frank pus requires drainage. Non-purulent fluid pH at or below 7.2 is high risk and usually needs an intercostal drain if safely accessible.
Pleural pH above 7.2 but below 7.4 is intermediate risk: consider drainage when LDH exceeds 900 IU/L and supportive features exist such as low glucose, large volume, CT pleural enhancement, septations or ongoing fever.
Cytology is the initial test for suspected secondary pleural malignancy, but a negative result must lead to further investigation when suspicion persists; do not use blind pleural biopsy.
Serum NT-proBNP can support heart failure in a unilateral effusion but must not be used alone; pleural NT-proBNP is not routinely needed because it is not superior to serum testing.
For therapeutic aspiration, drain slowly and generally no more than 1.5 L in one attempt; avoid vacuum bottles/wall suction and stop early if symptoms develop.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Raised hydrostatic pressure

Heart failure commonly produces bilateral transudative fluid as pulmonary and systemic venous pressures rise, although asymmetry or atypical features warrant further assessment.

Pleural inflammation

Pneumonia, tuberculosis, autoimmune disease and pulmonary embolism increase pleural vascular permeability, generating protein-rich exudative fluid, and its contribution is interpreted alongside the other recognised causes.

Malignancy

Primary or metastatic cancer obstructs lymphatic drainage or directly involves pleura, often causing recurrent unilateral exudative effusion.

Low oncotic or impaired lymphatic balance

Hypoalbuminaemia, cirrhosis, renal disease, chyle leakage and trapped lung alter filtration, absorption or pressure relationships and may produce bilateral or unusual fluid.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Fluid accumulation
Pleural fluid forms faster than parietal pleural lymphatics can remove it because filtration rises, permeability increases or drainage is obstructed.
2
Lung compression
The expanding collection reduces adjacent lung volume and may cause compressive atelectasis, particularly when large or rapidly developing.
3
Impaired ventilation
Reduced ventilation of compressed lung creates breathlessness and ventilation-perfusion mismatch, though symptom severity depends on size, tempo and cardiopulmonary reserve.
4
Pleural organisation
Persistent inflammation can deposit fibrin, form loculations and restrict lung expansion, complicating aspiration and definitive drainage.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Typical systemic transudate

Bilateral effusions with cardiomegaly, peripheral oedema/raised JVP and a convincing heart-failure picture, or clear renal/liver/hypoalbuminaemic disease, may improve with cause treatment without immediate aspiration.

Pleural infectionRed flag

Fever, pleuritic pain, pneumonia, sepsis, loculation or echogenic/septated fluid; frank pus or low pH moves from diagnosis to urgent antibiotics and drainage.

Malignant pattern

Unilateral recurrent fluid, weight loss, pleural nodularity/thickening, lung mass or previous cancer/asbestos exposure; negative cytology does not exclude malignancy.

HaemothoraxRed flag

Trauma/procedure, anticoagulation or malignancy with bloody fluid and shock/anaemia; compare pleural and peripheral haematocrit and obtain urgent surgical/pleural support.

Drainage complicationRed flag

New pain, persistent cough, dyspnoea, hypoxaemia or hypotension during rapid fluid removal may indicate re-expansion pulmonary oedema, pneumothorax or bleeding.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

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
Chest X-ray and thoracic ultrasoundFirst step
Why
Confirm amount/distribution, identify loculation and select a safe aspiration site.
Interpretation and limitations
Ultrasound assesses depth, diaphragm, lung and vessels in real time; mark and perform the procedure without changing position or delay that invalidates the site.
02
Diagnostic aspiration with paired serum protein and LDH
Why
Classify transudate versus exudate using Light's criteria.
Interpretation and limitations
Any one Light criterion makes the fluid an exudate. Diuresed heart failure can be misclassified; interpret with NT-proBNP and the whole picture.
03
Pleural pH, glucose, LDH, Gram stain/culture
Why
Stratify suspected parapneumonic effusion and identify infection.
Interpretation and limitations
Pus or pH at/below 7.2 generally requires drainage. For pH 7.2–7.4, LDH above 900 IU/L and clinical/radiological features strengthen the drainage case.
04
Pleural cytology
Why
Initial diagnosis of suspected secondary pleural malignancy.
Interpretation and limitations
Send an adequate 25–50 mL sample when possible. Sensitivity varies by tumour; a negative result with persistent suspicion needs tissue investigation.
05
Contrast-enhanced CT
Why
Assess pleura, lung, nodes, mediastinum and extra-thoracic disease and plan biopsy.
Interpretation and limitations
Pleural thickening/nodularity supports malignancy but a negative CT does not exclude it; interval CT may be appropriate when a benign diagnosis is assumed but malignancy remains possible.
06
Image-guided pleural biopsy or thoracoscopy
Why
Obtain tissue after non-diagnostic fluid or when cytology yield is likely low.
Interpretation and limitations
Choose image-guided cutting-needle biopsy or thoracoscopy according to target and patient; BTS advises against blind pleural biopsy.
07
Serum NT-proBNP
Why
Support suspected heart failure when a unilateral effusion creates diagnostic doubt.
Interpretation and limitations
A supportive result adds weight but is not standalone proof; dual pathology is common. Do not routinely duplicate with pleural NT-proBNP.
08
Targeted tests
Why
Resolve specific differential diagnoses.
Interpretation and limitations
Examples include pleural triglyceride/chylomicrons for chylothorax, haematocrit for haemothorax, ADA/IFN-gamma and tissue culture for TB in the right prevalence context, and amylase for oesophageal/pancreatic disease.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Pleural thickening

Diffuse pleural fibrosis can blunt the costophrenic angle and reduce movement without free fluid; ultrasound distinguishes solid thickening from an aspiratable collection.

Lower-lobe collapse or consolidation

Air-space opacity and volume loss may mimic fluid on a limited radiograph, while ultrasound or cross-sectional imaging clarifies lung versus pleural pathology.

Elevated hemidiaphragm

Diaphragmatic paralysis, abdominal distension or subphrenic disease can resemble basal opacity but lacks a pleural fluid pocket on ultrasound.

Haemothorax

Trauma, procedures or bleeding risk with high pleural-fluid haematocrit indicates blood rather than a routine serous effusion and requires urgent source assessment.

Empyema

Purulent fluid, loculation, low pleural pH or positive microbiology identifies pleural infection needing prompt drainage rather than diagnostic observation alone.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01BilateralSystemic-fluid patternFirst stepBilateral, relatively symmetric effusions with a convincing systemic cause and no pleural red flags.

1Assess heart, renal, liver and albumin status; obtain ECG, renal/liver tests, albumin and natriuretic peptide/echo as clinically indicated.
2Treat the systemic cause and monitor symptoms, weight, renal function and imaging response.
3Perform ultrasound-guided aspiration if effusions are very asymmetric, unilateral-dominant, associated with fever/pleuritic pain, lack cardiomegaly where expected, have a cancer/PE signal, or fail to improve as predicted.

02UnilateralUnexplained effusionNew unilateral or markedly asymmetric effusion without a secure benign explanation.

1Take history/examination, CXR and thoracic ultrasound; check FBC, CRP, renal/liver function, albumin and paired serum protein/LDH.
2Perform ultrasound-guided aspiration when safe. Send cytology, protein, LDH, glucose, pH and microbiology according to presentation; use blood-culture bottles for suspected infection.
3Obtain contrast CT—staging chest/abdomen/pelvis when malignancy is suspected—and review fluid with imaging in a pleural service.
4If cytology/fluid is non-diagnostic and concern persists, arrange image-guided pleural biopsy or thoracoscopy; reconsider PE, TB, lymphoma and chronic heart failure, and plan interval CT if malignancy is not fully excluded.

03InfectionParapneumonic decisionPneumonia/sepsis with pleural fluid and aspiration yields no frank pus.

1Measure pH immediately in a clean blood-gas syringe with no residual liquid heparin, avoiding local-anaesthetic contamination; send glucose, LDH and microbiology including blood-culture bottles.
2pH at/below 7.2: start/continue antibiotics and insert a small-bore intercostal drain if fluid volume and access are safe. Frank pus is drained without waiting for pH.
3pH above 7.2 but below 7.4: if LDH exceeds 900 IU/L and low glucose, large volume, enhancement, septation or ongoing fever is present, consider drainage. pH at/above 7.4 usually means no immediate drain.
4If no drain is placed, review closely and repeat aspiration/imaging when the patient fails to improve; do not let one borderline sample close the pathway.

04TherapeuticControlled symptom-relieving aspirationBreathlessness attributable to a sizeable effusion and aspiration is safe/appropriate.

1Consent, check anticoagulation/bleeding issues, use thoracic ultrasound and aseptic technique with adequate local anaesthesia.
2Drain slowly by manual syringe or gravity; do not use vacuum bottles or wall suction. Prescribe a volume/rate limit and observations.
3Generally stop by 1.5 L in one attempt and stop sooner for chest tightness/pain, persistent cough, worsening dyspnoea, desaturation or hypotension.
4If deterioration occurs, clamp/stop drainage, assess urgently for re-expansion oedema, pneumothorax or bleeding and obtain appropriate imaging/support.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

Local anaesthesia for aspiration, biopsy or drain insertion.

Lidocaine 1% for pleural procedure

Infiltrate skin, tract and parietal pleura incrementally; keep the total lidocaine dose at or below 3 mg/kg without adrenaline (1% solution contains 10 mg/mL) unless a lower product/patient limit applies.

Calculate and document the total from all syringes. Avoid intravascular injection; reduce in frailty/liver disease and monitor for circumoral tingling, tinnitus, seizures or arrhythmia.

Treats a cardiac-fluid-overload cause of bilateral effusions; it does not diagnose the cause or treat exudative pleural disease.

Furosemide for congestive fluid overload

A common starting dose in a diuretic-naive congested adult is 20–40 mg orally or IV; in acute heart failure already taking a loop diuretic, NICE advises an IV dose at least equivalent to the existing daily dose, then titrate to response.

Monitor BP, renal function, sodium, potassium, magnesium, weight and urine output. Do not repeatedly diurese an unexplained unilateral exudate instead of investigating it.

Treats associated hypoxaemia during assessment or drainage.

Controlled oxygen

Titrate to SpO2 94–98% for most adults, or 88–92% if at risk of hypercapnic respiratory failure.

Oxygen does not remove pleural fluid. A rising requirement during drainage is a stop-and-reassess signal.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Respiratory compromise

A large or rapidly accumulating effusion can substantially compress functioning lung and worsen oxygenation, particularly with limited baseline reserve.

Pleural infection

A parapneumonic collection may become infected and loculated, progressing to empyema, systemic sepsis and an organised pleural rind.

Trapped lung

Malignant or inflammatory visceral pleural restriction prevents re-expansion after drainage, causing persistent space, recurrent fluid and discomfort.

Procedure-related harm

Aspiration or drainage can cause pneumothorax, bleeding, infection or re-expansion injury, especially when anatomy, symptoms and drainage rate are not monitored.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

During therapeutic drainage, record BP, pulse, respiratory rate and SpO2 at the stated frequency and keep the maximum volume/rate visible at the bedside.
Stop drainage for pain, chest tightness, persistent cough, dyspnoea, desaturation or hypotension; urgently assess rather than simply slowing an unexplained deterioration.
After diagnostic aspiration, track cytology, microbiology and paired chemistry to a named clinician; arrange tissue/CT follow-up for non-diagnostic results.
For pleural infection, monitor fever, inflammatory markers, oxygen, drain output/patency and imaging; escalate persistent sepsis or residual collection to the pleural/surgical team.
For a presumed systemic cause, trend weight, fluid balance, renal/electrolyte values and radiographic response; failure to behave as expected reopens the diagnostic pathway.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Light's criteria maximise sensitivity

They deliberately identify exudates well but can label diuresed heart-failure fluid exudative; use the cardiac context and serum NT-proBNP rather than overriding the criteria casually.

pH is procedure-sensitive

Air, delay and local anaesthetic contamination can distort the result. Collect and analyse like a blood gas and interpret alongside imaging and clinical progress.

Negative cytology is not benign cytology

Yield depends on tumour biology. Persistent pleural thickening, nodules or recurrent unilateral fluid needs tissue despite a negative sample.

Bilateral does not equal harmless

Cancer, PE, autoimmune disease and infection can be bilateral; sample discordant or treatment-resistant cases.

Drainage has a dose

Volume, rate and symptoms matter. A written limit and observation schedule prevent uncontrolled offload from an unclamped drain.

11Common pitfallsFrequent interpretation and management errors.

01
Performing pleural aspiration without real-time thoracic ultrasound site selection.
02
Calling a unilateral effusion heart failure from BNP alone and missing dual pathology.
03
Treating a negative cytology result as exclusion of pleural malignancy.
04
Contaminating the pleural pH sample with lidocaine or delaying analysis.
05
Using wall suction/vacuum bottles for routine therapeutic aspiration or leaving drainage limits unspecified.
06
Applying Light's criteria without paired serum samples or without checking which laboratory upper LDH limit applies.