Pleural fluid: exudate or transudate (Light's criteria)

What it is

The pleural space normally holds only a thin film of fluid, filtered from the parietal pleura and drained by its lymphatics. An effusion forms either because the balance of pressures changes (a transudate: high capillary pressure in heart failure, low oncotic pressure in cirrhosis or nephrotic syndrome) or because the pleura itself is inflamed, infected or invaded and leaks protein and cells (an exudate: pneumonia, cancer, tuberculosis, pulmonary embolism, pancreatitis, autoimmune disease). The first question after a thoracentesis is which of the two it is, and Light's criteria answer it by comparing pleural fluid protein and lactate dehydrogenase (LDH) with serum values drawn at the same time. Meeting any one criterion makes the fluid an exudate. The criteria are very sensitive for exudates but label some transudates as exudates, especially in heart failure treated with diuretics; the serum-minus-pleural albumin gradient corrects this. A transudate points away from the pleura and towards treating the underlying cause; an exudate starts a second search with pH, glucose, cell differential, microbiology, cytology and specific tests such as adenosine deaminase (ADA), amylase and triglycerides.

Reference values and concepts

• Always send paired serum protein and LDH with the pleural sample; Light's criteria cannot be read without them.
• Normal pleural fluid: pH about 7.6, protein about 1–2 g/dL, few cells, glucose similar to blood.
• pH is measured in a blood-gas analyser, collected without air and kept cold; air or lidocaine in the syringe alters it.
• Low glucose (under 60 mg/dL) suggests complicated parapneumonic effusion or empyema, rheumatoid effusion, tuberculosis or cancer.
• Lymphocyte-predominant exudate: tuberculosis, cancer, lymphoma, chronic rheumatoid disease; ADA over about 40 U/L supports tuberculosis.
• Triglycerides over 110 mg/dL mean chylothorax; a pleural:blood haematocrit over 0.5 means haemothorax; high amylase suggests pancreatitis or oesophageal rupture.
• Cytology finds cancer in roughly six of ten malignant effusions; a negative result does not exclude it.

Reading it step by step

1. Look at the fluid — straw-coloured is common to both; pus means empyema; milky suggests chylothorax; frankly bloody suggests trauma, cancer or embolism.
2. Apply Light's criteria with paired serum values — any one positive criterion makes it an exudate.
3. If it is a borderline exudate on diuretics, check the albumin gradient — over 1.2 g/dL reclassifies it as a transudate.
4. Transudate: look for the cause in heart, liver and kidney — the pleura is not the problem; treat the cause.
5. Exudate with infection signs: read pH, glucose, Gram stain and culture — pH under 7.2, low glucose or pus mean drainage.
6. Read the cell differential — neutrophils suggest acute processes (pneumonia, embolism); lymphocytes suggest tuberculosis or cancer.
7. Read cytology and specific tests — ADA for tuberculosis, amylase for pancreas or oesophagus, triglycerides for chylothorax.
8. Put it with the chest image and the history — fever, weight loss, smoking, exposure and anticoagulation change the likely cause.

Pleural fluid patterns and likely causes (ranges vary by laboratory).

Patterns and common mistakes

• Forgetting the paired serum sample, so the fluid cannot be classified.
• Labelling a diuretic-treated heart failure effusion as an exudate and starting a search for cancer or tuberculosis.
• Waiting for culture in a parapneumonic effusion with pH under 7.2 or pus: drainage should not be delayed.
• Taking a negative cytology as proof there is no cancer; repeat cytology or pleural biopsy may be needed.
• In Colombia, missing tuberculosis in a lymphocytic exudate with high ADA; pleural culture is often negative.
• Sending pH in a syringe with air or local anaesthetic, which shifts the value.

Next step

• Transudate: treat the cause (heart failure, cirrhosis, nephrotic syndrome) and repeat drainage only for symptoms.
• Complicated parapneumonic effusion or empyema: start antibiotics and call the senior or thoracic team for a chest drain.
• Lymphocytic exudate with high ADA: start the tuberculosis work-up and consider pleural biopsy if cultures are negative.
• Unexplained exudate with negative cytology: repeat sampling, CT of the chest and referral for pleural biopsy or thoracoscopy.
• Haemothorax or rapidly reaccumulating fluid with breathlessness or low blood pressure needs urgent senior review.
• Review the chest image after the tap and follow the patient until the cause is clear and the effusion resolves.

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Elsewhere in DurgiPedia

• Pleural fluid: formation and drainage
• Starling forces at the capillary
• Pleural effusion syndrome
• Lactate dehydrogenase (LDH) as a lab marker
• Pleural effusion on the chest X-ray

Connections

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Thoracentesis (StatPearls (NCBI Bookshelf), 2024); Pleural Effusion (StatPearls (NCBI Bookshelf), 2024); Physiology, Pleural Fluid (StatPearls (NCBI Bookshelf), 2023).

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