Two kinds of pleural fluid
Fluid can collect between the lung and the chest wall for many reasons. The first step in finding the cause is to decide whether the fluid is a transudate, pushed out by pressure or low protein in the blood, or an exudate, produced by disease of the pleura itself. In 1972 Richard Light and colleagues proposed three simple tests on fluid drawn by thoracentesis and a blood sample taken at the same time.
The three criteria
The effusion is an exudate if at least one is met:
- Pleural fluid protein ÷ serum protein above 0.5
- Pleural fluid LDH ÷ serum LDH above 0.6
- Pleural fluid LDH above two-thirds of the upper limit of normal for serum LDH
If none is met, it is a transudate.
Worked example
Pleural protein 3.5 g/dL, serum protein 6.5 g/dL, pleural LDH 150 U/L, serum LDH 200 U/L, with a lab upper normal limit of 250 U/L:
| Result | Value |
|---|---|
| Protein ratio | 0.54 (above 0.5) |
| LDH ratio | 0.75 (above 0.6) |
| Pleural LDH as a share of the upper limit | 60% (below 67%) |
| Criteria met | 2 of 3: exudate |
| Serum minus pleural protein | 3.0 g/dL |
Common causes
| Transudate | Exudate |
|---|---|
| Heart failure (most common) | Pneumonia (parapneumonic effusion) |
| Cirrhosis with ascites | Cancer |
| Nephrotic syndrome | Pulmonary embolism |
| Peritoneal dialysis | Tuberculosis |
| Hypoalbuminemia | Autoimmune disease, pancreatitis |
When the criteria mislead
Light’s criteria are very sensitive for exudates, about 98%, but misclassify about 25% of transudates as exudates. This happens especially in heart failure patients treated with diuretics, which concentrate the pleural fluid. In that case:
- a serum minus pleural protein gradient above 3.1 g/dL, or
- a serum minus pleural albumin gradient above 1.2 g/dL
suggests the effusion is really a transudate. An elevated NT-proBNP in the fluid or blood also supports heart failure.
Next steps for an exudate
Further tests on the fluid usually include cell count and differential, glucose, pH, Gram stain and culture, cytology for cancer cells, and sometimes adenosine deaminase for tuberculosis. A pH below 7.2 in a parapneumonic effusion usually calls for drainage.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.