What is a pleural effusion and why separate transudates from exudates?
A pleural effusion is an abnormal accumulation of fluid in the pleural space — the thin cavity between the visceral and parietal pleura. Normally only a small film is present, about 0.1 to 0.3 mL/kg, produced from the parietal pleural vessels and drained by lymphatics that lie mainly in the dependent parts of the pleural cavity. When production rises or lymphatic drainage falls, fluid collects.
The first question in any effusion is: is this a transudate or an exudate? A transudate means the pleura itself is healthy and the fluid has been pushed out by altered pressures (hydrostatic or oncotic). An exudate means the pleura or its lymphatics are diseased — infection, inflammation, malignancy. The answer decides the work-up: a transudate usually needs only treatment of the systemic cause, while an exudate demands a search for the local cause.
What are Light's criteria and their cut-off values?
Light's criteria compare the pleural fluid with the serum drawn at about the same time. An effusion is exudative if one or more of the following is present:
| Criterion | Cut-off for exudate |
|---|---|
| Pleural fluid protein / serum protein ratio | More than 0.5 |
| Pleural fluid LDH / serum LDH ratio | More than 0.6 |
| Pleural fluid LDH (absolute) | More than two-thirds of the upper limit of normal serum LDH |
If none of the three criteria is met, the fluid is a transudate. The logic is simple: leaky, inflamed pleura lets protein and LDH (a marker of cell turnover and inflammation) enter the space, so fluid values approach serum values.
What is Heffner's modification and where do Light's criteria fail?
Heffner's criteria are a modification of Light's that do not need a paired serum sample. By Heffner's criteria an effusion is an exudate if one or more of these is present:
- Pleural fluid protein above 2.9 g/dL
- Pleural fluid cholesterol above 45 mg/dL
- Pleural fluid LDH above two-thirds of the upper limit of normal serum LDH
Light's criteria are very sensitive for exudates — about 98% in one 2023 study — but they are weaker at ruling out transudates, so some true transudates are labelled exudates. A 2026 study found that a pleural fluid-only combination of protein, LDH and cholesterol matched Light's accuracy without needing a serum sample, but Light's criteria remain the standard for exams. The classic situation is a patient with heart failure on diuretics: diuresis concentrates the pleural fluid, and in one cited study roughly one-fifth of patients with congestive cardiac failure on diuretics still met the exudate criteria.
When a transudate is suspected clinically but Light's criteria call it an exudate, the serum-effusion albumin gradient (SEAG) can help. It is serum albumin minus pleural fluid albumin; in the 2023 comparison study a gradient below 1.2 g/dL was used to classify an effusion as an exudate.
Which diseases cause transudative and which cause exudative effusions?
Transudates arise when hydrostatic or oncotic pressures change; exudates arise when pleural permeability rises or lymphatic drainage is blocked. StatPearls lists the common causes as follows.
| Type | Common causes | Mechanism |
|---|---|---|
| Transudate | Left heart failure, nephrotic syndrome, liver cirrhosis, hypoalbuminaemia, peritoneal dialysis | Raised hydrostatic pressure or reduced plasma oncotic pressure; fluid migration from the peritoneum in cirrhosis and dialysis |
| Exudate | Pneumonia, tuberculosis, malignancy, pancreatitis, lupus, rheumatoid arthritis, post-cardiac injury syndrome, chylothorax, haemothorax, post-CABG, benign asbestos effusion | Increased pleural permeability from infection or inflammation; lymphatic obstruction by tumour; rupture of thoracic vessels |
| Either / other | Pulmonary embolism (exudative or transudative), drugs (methotrexate, amiodarone, phenytoin, dasatinib), radiotherapy, oesophageal rupture, ovarian hyperstimulation syndrome | Varies with the cause |
How does a pleural effusion present and how is it seen on imaging?
Symptoms range from none to exertional breathlessness, cough, fever and pleuritic chest pain, depending on the cause. The amount of fluid correlates poorly with symptom severity; what matters most is the effect on chest-wall expansion. Pleuritic pain tends to ease once an effusion develops, while constant pain is a hallmark of malignant disease such as mesothelioma.
- Inspection / palpation: intercostal fullness in large effusions; decreased tactile vocal fremitus.
- Percussion: dullness over the fluid.
- Auscultation: decreased breath sounds; egophony is most pronounced at the upper edge of the effusion.
- A pleural rub can be heard in active pleurisy even without an effusion.
On an upright PA chest X-ray a meniscus sign and blunting of the costophrenic angle indicate a significant amount of fluid, typically more than 200 mL. A lateral decubitus film can detect as little as 50 mL. Ultrasound shows pleural fluid as hypoechoic; septations suggest a complex effusion such as a parapneumonic effusion or empyema, which needs drainage planning. Bedside ultrasound also improves the success of aspiration and lowers the risk of pneumothorax.
What do pleural fluid appearance, cells, pH and markers tell you?
After Light's criteria label the fluid an exudate, the details of the fluid point to the cause. StatPearls lists the routine tests after thoracentesis as fluid pH, protein, LDH, glucose, cell count with differential, Gram stain and culture, with cytology in selected cases.
| Finding | Think of |
|---|---|
| Lymphocyte-predominant exudate | Tuberculosis, post-CABG, rheumatoid arthritis, yellow nail syndrome, chylothorax, malignancy |
| Neutrophil-predominant exudate | Parapneumonic effusion |
| Eosinophilia (rare) | Pneumothorax, haemothorax, parasitic disease, drug-induced effusion |
| LDH above 1000 U/L | Tuberculosis, lymphoma, empyema |
| pH below 7.2 in pneumonia | Complex effusion — almost always needs chest tube drainage; low pH also seen in oesophageal rupture and rheumatoid arthritis |
| Triglycerides above 110 mg/dL, milky fluid | Chylothorax |
| Pleural fluid haematocrit more than 0.5 × serum haematocrit | Haemothorax |
| High amylase | Pancreatitis-related effusion |
| AFB smear, M. tuberculosis culture, adenosine deaminase | Ordered when tuberculosis is suspected |
For suspected malignant effusion, cytology is positive in roughly 40% to 60% on the first thoracentesis (StatPearls quotes about 60% sensitivity), rising to as much as 90% after three samples on separate days. If cytology is negative twice but malignancy is highly suspected, medical thoracoscopy with pleural biopsy is recommended.
How is a pleural effusion managed — thoracentesis, chest tube, pleurodesis?
Treatment aims first at the underlying cause. Drainage is recommended for symptomatic patients; in asymptomatic patients it is mainly diagnostic unless there are signs of haemorrhage or infection. Every new, undiagnosed effusion warrants a diagnostic thoracentesis, except when heart failure is confidently suspected — then a trial of diuresis may come first.
- Heart failure: diuretics; thoracentesis only if diuretics fail or the patient is significantly symptomatic.
- Complex parapneumonic effusion or empyema: chest tube drainage with antibiotics; intrapleural fibrinolytic and DNase can improve drainage; thoracoscopic decortication if these fail. Small-bore drains are as effective as larger ones and better tolerated.
- Malignant effusion: repeated drainage is not routine unless infection or severe symptoms arise; patients needing frequent drainage can have pleurodesis or a tunnelled catheter.
- Chylous effusion: initial conservative management; some need surgery.
How should you approach Light's criteria and effusion questions in the exam?
- Name the test first. Differentiating transudate from exudate = pleural fluid analysis by Light's criteria (protein ratio 0.5, LDH ratio 0.6, LDH 2/3 of serum upper limit).
- Count the thresholds. Exudate needs only one positive criterion; transudate needs none.
- Read the patient. Heart failure on diuretics, cirrhosis, nephrotic syndrome and hypoalbuminaemia point to transudate; pneumonia, tuberculosis, malignancy and pancreatitis point to exudate.
- Look for the discriminating fluid value. Triglyceride above 110 mg/dL = chylothorax; pH below 7.2 with pneumonia = chest tube; pleural haematocrit above half the serum value = haemothorax; lymphocytes + AFB/ADA = tuberculosis.
- Check the volume. More than 200 mL for a meniscus on a PA film; as little as 50 mL on lateral decubitus; tap no more than 1500 mL at a time.