exudate

If a transudate is plain water squeezed past intact walls, an exudate is what leaks when the walls themselves are damaged. Inflammation, infection or cancer makes the pleural blood vessels leaky, so the escaping fluid carries the larger molecules and cells that a healthy wall would have held back — it is protein-rich and often cloudy.

The leakiness comes from active disease at the pleural surface or nearby lung. Pneumonia can spill inflammatory fluid into the pleura (a parapneumonic effusion, which may turn into pus, or empyema); cancer cells seeding the pleura attract fluid; pulmonary embolism, tuberculosis, pancreatitis and autoimmune diseases all produce exudates. Because the list is long, an exudate almost always demands further work to pin down the cause.

Labeling an effusion as exudative is a starting point, not a diagnosis. The fluid is then examined more closely — its cell counts, glucose, pH, microbiology and cytology — and imaging or biopsy may follow. Some exudates, especially infected ones, need urgent drainage rather than fluid that can wait.