Pleural Fluid Analysis and Light's Criteria
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Direct answer
Pleural fluid analysis classifies an effusion as an exudate or transudate, usually by Light's criteria, and then narrows the cause using pH, glucose, lactate dehydrogenase, adenosine deaminase and cytology. An effusion is an exudate if the pleural-to-serum protein ratio exceeds 0.5, the pleural-to-serum lactate dehydrogenase ratio exceeds 0.6, or pleural lactate dehydrogenase exceeds two-thirds of the upper limit of normal serum value. Meeting any one of these criteria is enough to label an effusion an exudate.
What you must remember
- Light's criteria: any one of three ratios marks an exudate, and the criteria are applied to every effusion before a cause is named.
- Transudate: typical causes are heart failure, cirrhosis and nephrotic syndrome, and transudates are often bilateral and clear.
- Complicated parapneumonic effusion: pleural pH below 7.2 signals the need for drainage, and pH below 7.0 indicates more severe infection.
- Glucose: below 60 mg/dL suggests complicated parapneumonic infection, tuberculosis, malignancy or rheumatoid pleurisy.
- Adenosine deaminase: above about 40 U/L in a lymphocyte-dominant exudate supports tuberculous pleurisy in an endemic setting, though lymphoma and empyema can also raise it.
- Cytology: malignant cells confirm a malignant effusion, but sensitivity is limited, so repeat sampling and pleural biopsy follow a negative result.
- Amylase: high pleural amylase suggests pancreatic disease or oesophageal rupture.
- Eosinophils: above 10 percent suggests air or blood in the pleural space, a drug reaction or a parasitic cause rather than infection.
Reading the panel as a pattern
A 45-year-old man with fever and right lower zone dullness has a pleural tap that shows turbid fluid. Single values rarely settle the diagnosis, so the panel is read as a set of findings that each point to a cause.
| Pleural finding | Points towards | Practical next step |
|---|---|---|
| pH below 7.2 | Complicated parapneumonic effusion or empyema | Chest drain with antibiotics |
| Glucose below 60 mg/dL | Infection, tuberculosis, malignancy or rheumatoid pleurisy | Culture and cytology of the fluid |
| ADA above about 40 U/L | Tuberculous pleurisy in a lymphocytic exudate | Pleural biopsy and tuberculosis workup |
| High amylase | Pancreatic disease or oesophageal rupture | Imaging of pancreas or oesophagus |
| Eosinophils above 10 percent | Air, blood, drugs or parasites | Check for pneumothorax, haemothorax and drug history |
| Lymphocyte dominance | Tuberculosis, lymphoma or malignancy | Cytology and pleural biopsy |
A pH below 7.2 with a low glucose in a febrile patient is a drain-and-antibiotics picture, whereas lymphocytic fluid with raised ADA sends the workup towards tuberculosis. The table is a sequence to check, not a single answer.
Where the exam tries to catch you
Pleural questions reward the candidate who reads the fluid before naming the disease.
- Trap: assuming every effusion in a cancer patient is malignant Correct: parapneumonic, tuberculous and drug-related effusions are common too, and a negative cytology does not exclude malignancy.
- Trap: treating a low pleural pH with antibiotics alone Correct: pH below 7.2 means a complicated effusion that usually needs drainage as well.
- Trap: taking a raised ADA as proof of tuberculosis Correct: ADA is raised in lymphoma and empyema too, so it supports rather than proves the diagnosis, and pleural biopsy confirms it.
Frequently asked questions
What are Light's criteria for an exudate?
An effusion is an exudate if the pleural-to-serum protein ratio exceeds 0.5, the pleural-to-serum lactate dehydrogenase ratio exceeds 0.6, or pleural lactate dehydrogenase exceeds two-thirds of the upper limit of normal serum value. Meeting any one criterion classifies the effusion as an exudate.
Why is pleural pH measured in parapneumonic effusion?
Pleural pH measures the acidity of infected fluid and predicts whether drainage is needed. A pH below 7.2 signals a complicated parapneumonic effusion that usually requires a chest drain alongside antibiotics.
Which pleural fluid findings suggest tuberculous pleurisy?
A lymphocyte-dominant exudate with adenosine deaminase above about 40 U/L suggests tuberculous pleurisy in an endemic setting. Pleural biopsy is the reliable confirmatory test, since culture and PCR of pleural fluid have low yield.
What does a high pleural amylase suggest?
High pleural amylase suggests pancreatic disease, such as pancreatitis or a pancreatic pseudocyst, or oesophageal rupture. Pleural amylase should be interpreted together with the clinical picture and imaging.