Characterization of Pleural Fluid
| Transudate | Modified Transudate | Non-septic Exudate | Septic Exudate | Chyle | Hemorrhage |
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| Color | Colorless to pale yellow | Yellow or pink | Yellow or pink | Yellow to red-brown | Milky white | Red | | Turbidity | Clear | Clear to cloudy | Clear to cloudy; fibrin | Cloudy to opaque; fibrin | Opaque | Opaque | | Protein (g/dL) | <1.5 | 2.5–5.0 | 3.0–8.0 | 3.0–7.0 | 2.5–6.0 | 3.0 | | Nucleated cells/µL | <1,000 | 1,000–7,000 (LSA up to 100,000) | 5,000–20,000 (LSA up to 100,000) | 5,000–300,000 | 1,000–20,000 | Similar to peripheral blood | | Cytology | Mostly mesothelial cells and macrophages | Mostly macrophages and mesothelial cells; few nondegenerate PMNs; neoplastic cells in some cases | Mostly nondegenerate PMNs and macrophages; neoplastic cells in some cases | Mostly degenerate PMNs; also macrophages; bacteria | Small lymphocytes, PMNs, and macrophages | Mostly RBCs; with erythrophagocytosis | | Disease associations | Hypoalbuminemia (protein-losing nephropathy, protein-losing enteropathy, or liver disease); early CHF | CHF; neoplasia; diaphragmatic hernia; pancreatitis | FIP; neoplasia; diaphragmatic hernia; lung lobe torsion | Pyothorax | Lymphangiectasia, CHF, cranial vena cava obstruction, neoplasia, fungal, dirofilariasias, diaphragmatic hernia, lung lobe torsion, trauma | Trauma, coagulopathy, neoplasia, lung lobe torsion |
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Modified from Sherding RG. Diseases of the pleural cavity. In: Sherding RG, ed. The Cat: Diseases and Clinical Management, 2nd ed. New York: Churchill Livingstone, 1994, p. 1061.