What an effusion is, and why it matters
A pleural effusion is an abnormal build-up of fluid in the pleural space. A small amount may go unnoticed; a larger one compresses the lung, causing breathlessness, a dull ache, and sometimes a dry cough. On exam, the area is dull to percussion and quiet on auscultation because fluid muffles breath sounds.
The job is not just to drain it but to understand it. The single most useful classification splits effusions into two families by how the fluid got there: a transudate (pushed across intact membranes by pressure imbalance) or an exudate (leaked across membranes made leaky by inflammation, infection, or malignancy).
Transudate vs exudate: pressure vs leakiness
A transudate is essentially filtered plasma water with little protein. It accumulates when hydrostatic pressure is high or oncotic pressure is low — most often from heart failure, where backed-up pressure pushes fluid out (closely related to cardiogenic pulmonary edema), but also from cirrhosis or nephrotic syndrome. Transudates point you toward a systemic problem, not the pleura itself.
An exudate is protein-rich and cell-rich because the membranes themselves are leaking. Think infection (pneumonia with a parapneumonic effusion, or empyema), malignancy, pulmonary embolism, tuberculosis, and inflammatory disease. Exudates say: the problem is local to the pleura or lung — look harder.
Light's criteria: turning labs into an answer
To tell the two apart you sample the fluid by thoracentesis (often guided by pleural ultrasound) and send it with a paired blood sample for fluid analysis. Light's criteria compare pleural fluid to serum. The fluid is an exudate if any one of the three is met; if none is met, it is a transudate.
LIGHT'S CRITERIA — exudate if ANY one is true: 1. Pleural protein / serum protein > 0.5 2. Pleural LDH / serum LDH > 0.6 3. Pleural LDH > 2/3 of upper limit of normal serum LDH Worked example Serum protein 7.0 g/dL Pleural protein 4.2 g/dL Serum LDH 200 U/L Pleural LDH 260 U/L Upper limit of normal serum LDH = 225 U/L 1. 4.2 / 7.0 = 0.60 > 0.5 -> EXUDATE criterion met 2. 260 / 200 = 1.30 > 0.6 -> EXUDATE criterion met 3. 2/3 x 225 = 150; 260 > 150 -> EXUDATE criterion met Result: EXUDATE (all three met -> investigate pleura/lung)