What is Light's criteria and why does it matter?

Light's criteria is a three-part biochemical test that separates transudates from exudates after a diagnostic thoracentesis. An effusion is classified as exudative if any one condition is met: pleural protein/serum protein >0.5, pleural LDH/serum LDH >0.6, or pleural LDH >2/3 the upper limit of normal serum LDH. The test has approximately 99% sensitivity for exudates — it rarely misses one. The practical limitation is specificity: about one in four true transudates (especially in patients on diuretics for heart failure) will misclassify as exudate. When this is suspected, a serum–pleural albumin gradient greater than 12 g/L or a pleural NT-proBNP above 1,500 pg/mL points back to a cardiac origin.

What is a malignant pleural effusion and how is it diagnosed?

A malignant pleural effusion (MPE) is pleural fluid with confirmed malignant cells on cytology or pleural biopsy, or strongly characteristic features in a patient with known cancer. Lung (approximately 35%), breast (approximately 25%), and lymphoma (approximately 10%) are the commonest primaries in Australia. A single thoracentesis yields positive cytology in only about 60% of cases — sending 50–100 mL with a cell-block preparation and immunohistochemistry raises this to approximately 85% on a second sample. When cytology is negative and suspicion remains, image-guided percutaneous pleural biopsy achieves roughly 85% sensitivity, and medical thoracoscopy approximately 95%.

What is an indwelling pleural catheter and how does it work?

An indwelling pleural catheter (IPC) — commercial brands include PleurX and Rocket Aspira — is a tunnelled silicone catheter inserted under local anaesthesia as a day procedure by a respiratory physician or interventional radiologist. The patient or a district nurse drains fluid at home via vacuum bottle two to three times per week. In the TIME2 and AMPLE trials, IPC produced equivalent dyspnoea relief to talc pleurodesis while reducing hospital days from around 17 to around 10. Approximately 50% of patients achieve spontaneous auto-pleurodesis within six to twelve weeks, allowing catheter removal. Complications include catheter-site infection in approximately 5% and tract metastasis in approximately 5% with mesothelioma.

What does 'trapped lung' mean and why does it affect treatment choice?

Trapped lung occurs when the visceral pleura becomes fibrotically encased, preventing the lung from fully expanding after drainage. When this happens, a negative pressure develops in the pleural space, drainage becomes uncomfortable, and talc pleurodesis cannot work because the two pleural surfaces cannot appose. An indwelling pleural catheter provides ongoing drainage and symptom relief without requiring full lung re-expansion, making it the preferred option when trapped lung is identified on imaging or suspected clinically. An unexpandable lung visible on post-drainage chest X-ray, or drainage that stops after removing only a small volume with a sudden pressure drop, suggests the diagnosis.

When should a GP refer for pleural disease and what should the referral include?

A new pleural effusion should be referred urgently to respiratory medicine — within one to two weeks for a stable patient, and on the same day for haemodynamic compromise or suspected empyema. The referral should note the clinical context (is the patient on diuretics? known malignancy? fever?), include the chest X-ray or ultrasound report, bloods (FBC, UEC, LFT, NT-proBNP, albumin), and any relevant history of asbestos exposure, travel to TB-endemic regions, or recent procedures. Pushing for a full diagnostic panel on the first tap avoids a repeat procedure.

Pleural effusion

Pleural effusion workup: Light's criteria and malignant effusion management

By Dr HB Lo, FACRRM Reviewed 3 October 2026 9 min read

A pleural effusion is excess fluid in the pleural space. Diagnosis pivots on separating transudates from exudates using Light's criteria (exudate if any: pleural/serum protein >0.5; pleural/serum LDH >0.6; or pleural LDH >2/3 ULN). Sensitivity ≈99%; ≈25% of diuresed heart failure effusions misclassify — correct with serum–pleural albumin gradient or pleural NT-proBNP.

Malignant pleural effusion (MPE) complicates ≈40% of advanced cancers. An indwelling pleural catheter (IPC) plus intrapleural talc is the preferred strategy when prognosis exceeds three months, achieving pleurodesis in ≈43% at five weeks.

Fluid around the lung — found incidentally on imaging, or presenting with breathlessness, pleuritic chest pain, or a dull percussion note — is a common general practice diagnostic challenge. The effusion itself is not a diagnosis; it is a clue to the underlying condition, and the first diagnostic tap is the single best opportunity to identify it.

This article focuses on the two areas most likely to change patient outcomes in the general practice context: the systematic biochemical workup framework, and the management of malignant pleural effusion (MPE), which requires urgent multidisciplinary input.

A. Core clinical — the AU general practice framework

Why fluid accumulates

The pleural space normally contains approximately 10 mL of low-protein lubricating fluid, maintained by a balance between parietal capillary filtration and parietal lymphatic absorption. Effusion arises from five pathophysiological mechanisms: increased hydrostatic pressure (heart failure), reduced oncotic pressure (cirrhosis, nephrotic syndrome, hypoalbuminaemia), increased capillary permeability (infection, inflammation, malignancy, pulmonary embolism), decreased lymphatic drainage (malignancy, mediastinal obstruction), or communication with another fluid space (hepatic hydrothorax via peritoneal-pleural flow, dialysate leak, chylothorax from thoracic duct disruption).

Separating transudates (mechanisms 1–2) from exudates (mechanisms 3–5) is the pivot of the diagnostic algorithm because it focuses the differential to a manageable list and dictates whether further pleural-space workup is necessary.

Light’s criteria — the diagnostic standard

Per Light Ann Intern Med 1972, an effusion is classified as exudative if any one of the following is met:

Pleural protein / serum protein >0.5, OR
Pleural LDH / serum LDH >0.6, OR
Pleural LDH >2/3 the upper limit of normal serum LDH.

If none are met, it is a transudate. Sensitivity for exudate is approximately 99% — Light’s criteria rarely misses a true exudate. The practical limitation is specificity: around 25% of true transudates misclassify as exudate, most commonly in patients with heart failure who have been treated with loop diuretics before the tap (diuretics concentrate pleural protein and LDH relative to serum).

When Light’s classifies an effusion as exudative but the clinical picture strongly suggests a cardiac cause, two corrections are endorsed by the BTS 2023 Pleural Disease Guideline:

Serum–pleural albumin gradient >12 g/L → transudate (despite Light’s).
Pleural NT-proBNP >1,500 pg/mL → cardiac origin with approximately 95% sensitivity and specificity per Porcel Chest 2006.

What to send on every diagnostic tap

The first tap is the single best opportunity. BTS 2023 and the Thoracic Society of Australia and New Zealand recommend ordering everything in parallel rather than waiting for Light’s result before sending further tests:

Investigation	Purpose
Plain biochemistry — protein, LDH, glucose, albumin	Light’s criteria + empyema screen (glucose below 2.2 mmol/L)
pH (heparinised arterial-blood-gas syringe, ice, analyse within 1 hour)	pH below 7.2 indicates drainage in parapneumonic effusion
Cytology — 50–100 mL, cell-block + immunohistochemistry	Malignancy; sensitivity ~60% single tap, ~85% with second
Cell count + differential	Neutrophilic (parapneumonic), lymphocytic (TB, malignancy), eosinophilic (PE, drug, post-CABG)
Gram + culture, aerobic + anaerobic	Bacterial infection — inoculate blood-culture bottles at bedside for higher yield
AFB smear + mycobacterial culture + TB PCR	Tuberculous pleurisy
Adenosine deaminase (ADA)	TB pleurisy (cutoff >40 U/L; sensitivity approximately 90%, specificity approximately 90%)
Amylase	Pancreatic effusion or oesophageal rupture (Boerhaave — salivary isoenzyme)
Triglycerides + cholesterol	Chylothorax (TG >1.24 mmol/L) or pseudochylothorax
NT-proBNP	Cardiac origin when Light’s misclassifies diuresed heart failure

Ultrasound guidance is mandatory for every thoracentesis. BTS 2023 rates this as Class I evidence — ultrasound guidance reduces iatrogenic pneumothorax by approximately 50% and increases procedure success. All pleural procedures should be image-guided.

Differential diagnosis

Transudates account for more than half of all effusions in Australia. Heart failure is the single most common cause, typically producing bilateral effusions that are right-sided or equal. Cirrhotic hepatic hydrothorax is right-sided in approximately 70% and may occur without clinically evident ascites. Nephrotic syndrome and hypoalbuminaemia of any cause typically produce small bilateral effusions.

Exudates have a broader differential: parapneumonic effusion and empyema (managed urgently with drainage per eTG Respiratory), malignancy, pulmonary embolism (small ipsilateral effusion with haemorrhagic fluid), TB pleurisy (lymphocyte-predominant, elevated ADA), connective tissue disease (rheumatoid arthritis produces very low glucose and pH), post-cardiac surgery, and drug-induced effusions (amiodarone, methotrexate, immune-checkpoint inhibitors).

In Australia, asbestos-related benign pleural effusion remains an important diagnosis of exclusion — it carries lifelong mesothelioma surveillance obligations and access to compensation pathways.

B. Malignant pleural effusion — diagnosis and workup

Malignant pleural effusion complicates approximately 40% of advanced cancers. The most common primaries in Australia are lung (approximately 35%), breast (approximately 25%), lymphoma (approximately 10%), ovarian (approximately 5%), and gastrointestinal cancers.

Diagnostic yield

A single thoracentesis with cytology achieves a positive result in approximately 60% of confirmed MPE cases. Sending 50–100 mL minimum with cell-block processing and immunohistochemistry raises this to approximately 85% on a second sample, per BTS 2023.

When cytology is negative and clinical suspicion remains, escalate to image-guided percutaneous pleural biopsy (sensitivity approximately 85% for diffuse pleural disease) or medical thoracoscopy / VATS pleural biopsy (sensitivity approximately 95%), which also allows therapeutic talc poudrage at the same procedure.

The LENT score (pleural LDH, ECOG performance status, neutrophil-lymphocyte ratio, tumour type) stratifies median survival from approximately one month (high-risk) to approximately twelve months (low-risk) and informs the choice between a minimally invasive versus a more definitive management strategy.

C. Malignant pleural effusion — the management ladder

The ATS/STS/STR 2018 statement and BTS 2023 define four main management options:

Therapeutic thoracentesis

A single tap to drain symptomatic fluid, with no attempt at pleurodesis. Appropriate when life expectancy is very short, the patient declines invasive options, or as an initial response assessment to confirm that dyspnoea is driven by the effusion. Volume is capped at 1.5 L per tap (re-expansion pulmonary oedema risk). Recurrence approaches 98% by 30 days — therapeutic thoracentesis alone is rarely adequate if life expectancy exceeds one month.

Indwelling pleural catheter (IPC)

A tunnelled silicone catheter (PleurX, Rocket Aspira) inserted under local anaesthesia as a day procedure. The patient or district nurse drains via vacuum bottle two to three times weekly at home. In the TIME2 trial (Davies JAMA 2012), IPC versus talc pleurodesis produced equivalent dyspnoea relief with a mean reduction in hospital days from 17 to 10. The AMPLE trial (Thomas JAMA 2017) confirmed equivalent quality of life outcomes. Approximately 50% of patients achieve spontaneous auto-pleurodesis within six to twelve weeks, allowing catheter removal.

In Australia, IPCs are widely available at tertiary respiratory and cancer centres. Community drainage is supported through district nursing services or patient and family self-drainage, which centres actively teach.

Complications include catheter-related infection in approximately 5% and tract metastasis in approximately 5% (particularly with mesothelioma).

Talc pleurodesis

Chemical obliteration of the pleural space via talc slurry (4–5 g in 50 mL saline instilled via chest drain, clamped one hour, then drained; bedside procedure, approximately 70% success) or thoracoscopic talc poudrage (direct insufflation under vision; approximately 75–80% success). Requires lung re-expansion and absence of trapped lung. Always use large-particle graded talc — small-particle talc historically caused ARDS.

IPC plus intrapleural talc — the modern preferred approach

IPC-PLUS (Bhatnagar NEJM 2018) randomised 154 patients to IPC alone versus IPC plus talc slurry instilled via the catheter at day ten. Pleurodesis at 35 days: 43% versus 23% (NNT approximately five), with no excess adverse events. BTS 2023 now recommends this combination as the preferred strategy for patients with prognosis exceeding three months and no trapped lung.

D. Australian operations

Referral and specialist pathway

A new pleural effusion warrants urgent respiratory medicine referral — within one to two weeks for a stable patient, same day for haemodynamic compromise, high-grade fever suggesting empyema, or large rapidly accumulating effusion causing respiratory distress. For known malignancy with a new symptomatic effusion, a joint respiratory and oncology referral facilitates coordinated management.

IPC-capable centres in Australia include all major tertiary respiratory units: Royal Prince Alfred (Sydney), St Vincent’s (Sydney), Alfred (Melbourne), Austin (Melbourne), Royal Melbourne, Princess Alexandra (Brisbane), Royal Brisbane and Women’s, Sir Charles Gairdner (Perth), Royal Adelaide, and Royal Hobart.

Mesothelioma multidisciplinary teams and asbestos-related disease compensation pathways are available at these centres and through state WorkCover and asbestos disease support agencies.

MBS items for referral and investigation

General practice attendance: MBS 23/36/44. Chest ultrasound: MBS 55054. CT chest with contrast: MBS 56507/56807. Respiratory specialist consultation: MBS 105/106. Diagnostic thoracentesis (image-guided, specialist): MBS 38812. Therapeutic thoracentesis or IPC insertion (specialist): MBS 38815. VATS pleural biopsy or pleurodesis: MBS 30614/30615. Pleural fluid biochemistry, cytology and microbiology panel: MBS 73529. Chronic disease management plan: MBS 132/133.

Note: IPC drainage bottles (PleurX, Rocket Aspira) are not PBS-listed. They are typically supplied through hospital programmes. Some state cancer services and private health insurers fund ongoing home drainage supplies.

E. Special populations

Pregnancy. Pleural effusion in pregnancy is uncommon but serious — pulmonary embolism, pre-eclampsia with hepatic involvement, and peripartum cardiomyopathy are specific causes to consider. Diagnostic tap is safe in pregnancy with ultrasound guidance. Management decisions require obstetric and respiratory co-care.

Aboriginal and Torres Strait Islander patients. Tuberculosis remains more prevalent in certain Aboriginal and Torres Strait Islander communities; ADA and mycobacterial cultures are particularly important when assessing exudative effusions in this context. Mesothelioma rates attributable to natural asbestos exposure (particularly in the Wittenoom cohort and some WA communities) require culturally safe discussion of compensation pathways. MBS 715 health assessment provides an entry point for comprehensive review.

Older adults and frailty. Therapeutic goals shift when functional status is limited. Comfort-directed drainage (thoracentesis as needed) rather than IPC may better align with patient goals in the final months of life. Early palliative care integration improves quality of life in advanced cancer with MPE, as shown in the Temel NEJM 2010 trial.

Immunocompromised patients. HIV-positive patients and those on immune-checkpoint inhibitors have expanded differentials including opportunistic infections (Pneumocystis pleuritis, cryptococcal pleuritis) and immune-related pleural toxicity respectively.

When to escalate

Refer urgently or to the emergency department for:

Large effusion causing haemodynamic compromise or severe respiratory distress
Suspected empyema (fever, purulent-appearing fluid, pH below 7.2) — requires emergency drainage
Suspected haemothorax (post-trauma or suspected ruptured aortic aneurysm)
New bilateral effusions with papilloedema or hypertensive emergency
MPE with rapid deterioration in visual acuity or dyspnoea refractory to drainage

Refer semi-urgently to respiratory medicine for:

Any new unexplained exudative effusion
Asbestos exposure history with new pleural disease
Lymphocyte-predominant effusion with elevated ADA (TB workup)
Suspected malignant effusion based on clinical context

What this article is and is not

This is general health information drawn from the BTS 2023 Pleural Disease Guideline, Thoracic Society of Australia and New Zealand, Therapeutic Guidelines (eTG), and key clinical trials (TIME2, AMPLE, IPC-PLUS, Light 1972). It does not constitute personal medical advice and does not create a doctor–patient relationship. Management decisions — including the choice between IPC, talc pleurodesis, and combination approaches — require individual clinical assessment by specialist respiratory physicians and oncologists.

For Australian consumer-friendly information: HealthDirect, Lung Foundation Australia, Cancer Council Australia.

Sources cited

Frequently asked questions

What is Light's criteria and why does it matter?

Light's criteria is a three-part biochemical test that separates transudates from exudates after a diagnostic thoracentesis. An effusion is classified as exudative if any one condition is met: pleural protein/serum protein >0.5, pleural LDH/serum LDH >0.6, or pleural LDH >2/3 the upper limit of normal serum LDH. The test has approximately 99% sensitivity for exudates — it rarely misses one. The practical limitation is specificity: about one in four true transudates (especially in patients on diuretics for heart failure) will misclassify as exudate. When this is suspected, a serum–pleural albumin gradient greater than 12 g/L or a pleural NT-proBNP above 1,500 pg/mL points back to a cardiac origin.
What is a malignant pleural effusion and how is it diagnosed?

A malignant pleural effusion (MPE) is pleural fluid with confirmed malignant cells on cytology or pleural biopsy, or strongly characteristic features in a patient with known cancer. Lung (approximately 35%), breast (approximately 25%), and lymphoma (approximately 10%) are the commonest primaries in Australia. A single thoracentesis yields positive cytology in only about 60% of cases — sending 50–100 mL with a cell-block preparation and immunohistochemistry raises this to approximately 85% on a second sample. When cytology is negative and suspicion remains, image-guided percutaneous pleural biopsy achieves roughly 85% sensitivity, and medical thoracoscopy approximately 95%.
What is an indwelling pleural catheter and how does it work?

An indwelling pleural catheter (IPC) — commercial brands include PleurX and Rocket Aspira — is a tunnelled silicone catheter inserted under local anaesthesia as a day procedure by a respiratory physician or interventional radiologist. The patient or a district nurse drains fluid at home via vacuum bottle two to three times per week. In the TIME2 and AMPLE trials, IPC produced equivalent dyspnoea relief to talc pleurodesis while reducing hospital days from around 17 to around 10. Approximately 50% of patients achieve spontaneous auto-pleurodesis within six to twelve weeks, allowing catheter removal. Complications include catheter-site infection in approximately 5% and tract metastasis in approximately 5% with mesothelioma.
What does 'trapped lung' mean and why does it affect treatment choice?

Trapped lung occurs when the visceral pleura becomes fibrotically encased, preventing the lung from fully expanding after drainage. When this happens, a negative pressure develops in the pleural space, drainage becomes uncomfortable, and talc pleurodesis cannot work because the two pleural surfaces cannot appose. An indwelling pleural catheter provides ongoing drainage and symptom relief without requiring full lung re-expansion, making it the preferred option when trapped lung is identified on imaging or suspected clinically. An unexpandable lung visible on post-drainage chest X-ray, or drainage that stops after removing only a small volume with a sudden pressure drop, suggests the diagnosis.
When should a GP refer for pleural disease and what should the referral include?

A new pleural effusion should be referred urgently to respiratory medicine — within one to two weeks for a stable patient, and on the same day for haemodynamic compromise or suspected empyema. The referral should note the clinical context (is the patient on diuretics? known malignancy? fever?), include the chest X-ray or ultrasound report, bloods (FBC, UEC, LFT, NT-proBNP, albumin), and any relevant history of asbestos exposure, travel to TB-endemic regions, or recent procedures. Pushing for a full diagnostic panel on the first tap avoids a repeat procedure.

Source quality

Sources grouped by evidence tier. AU primary tier first; international where AU is silent or lagging; named-author reconstruction where guidelines have not yet caught up. How tiers work.

T1 AU primary 6 sources
T2 International primary 2 sources
- BTS 2023 Pleural Disease Guideline
- ATS/STS/STR Statement on Malignant Pleural Effusion 2018
T3 Named-author reconstruction 5 sources