Transudate from pressure change; exudate from increased permeability
Expansion
| Transudate | Exudate | |
|---|---|---|
| Mechanism | Altered hydrostatic or oncotic pressure | Increased permeability |
| Protein | Low (under 30 g/l) | High (over 30 g/l) |
| Specific gravity | Under 1.012 | Over 1.020 |
| LDH | Low | High |
| Cells | Few | Many |
| Causes | Heart failure, cirrhosis, nephrotic syndrome | Infection, malignancy, infarction, inflammation |
Light’s criteria identify a pleural exudate if any one holds: fluid to serum protein ratio above 0.5, fluid to serum LDH above 0.6, or fluid LDH above two-thirds the upper limit of normal serum LDH. They are deliberately weighted to avoid missing an exudate.
For ascites, the serum-ascites albumin gradient is more useful than the exudate and transudate distinction: a gradient of 11 g/l or more indicates portal hypertension, and below that suggests peritoneal disease such as malignancy, tuberculosis or pancreatitis.
Naming by site is worth keeping straight: effusion in a body cavity, ascites in the peritoneum, anasarca for generalised severe oedema, and hydrothorax, hydropericardium and hydrocephalus for the respective spaces.