Exudate or transudate first, then the specific tests
Mnemonic
Light’s criteria: the fluid is an exudate if any one applies.
- Pleural protein to serum protein ratio over 0.5
- Pleural LDH to serum LDH ratio over 0.6
- Pleural LDH more than two thirds of the upper limit of normal serum LDH
“Two thirds, one half, two thirds” covers the three numbers: 0.5 protein, 0.6 LDH, and two thirds of the upper limit.
Transudates are pressure problems (heart failure, cirrhosis, nephrotic syndrome); exudates are permeability problems (infection, malignancy, embolism, inflammation).
“pH under 7.2 means a drain” is the single most actionable number in an infected effusion, alongside frank pus or organisms on Gram stain.
Light’s criteria misclassify about a quarter of transudates as exudates, particularly in diuresed heart failure; a serum to pleural albumin gradient over 12 g/l reclassifies them correctly.
Expansion
Light’s criteria: the fluid is an exudate if any one applies
- Pleural protein to serum protein ratio over 0.5
- Pleural LDH to serum LDH ratio over 0.6
- Pleural LDH more than two thirds of the upper limit of normal serum LDH
Transudate causes, driven by pressure and oncotic forces: heart failure, cirrhosis, nephrotic syndrome, hypoalbuminaemia, peritoneal dialysis, hypothyroidism, Meigs syndrome, constrictive pericarditis.
Exudate causes, driven by inflammation and increased permeability: infection (parapneumonic, empyema, tuberculosis), malignancy, pulmonary embolism, connective tissue disease, pancreatitis, post-cardiac injury syndrome, drugs, asbestos.
Light’s criteria are highly sensitive for exudates but misclassify about a quarter of transudates, particularly in heart failure treated with diuretics, which concentrates the protein. Where the clinical picture suggests a transudate, a serum to pleural albumin gradient over 12 g/l reclassifies it correctly.
Additional tests
| Test | Significance |
|---|---|
| pH under 7.2 | Complicated parapneumonic effusion, needs a chest drain |
| Glucose low | Empyema, tuberculosis, rheumatoid, malignancy |
| Amylase high | Pancreatitis, oesophageal rupture, malignancy |
| Triglycerides over 1.24 mmol/l | Chylothorax |
| Frank pus, or organisms on Gram stain | Empyema, drain regardless of pH |
| Cytology | Malignancy, positive in about 60 per cent overall |
| Adenosine deaminase | Tuberculosis |
Appearance is informative: straw coloured is typical, turbid or purulent suggests empyema, milky suggests chyle, blood stained suggests malignancy, embolism or trauma, and food particles indicate oesophageal rupture.