Pleural effusion, the accumulation of fluid between the lung and chest wall, can arise from multiple causes, including heart failure, infection, malignancy, and liver disease. Determining whether an effusion is transudative or exudative is essential because it guides diagnostic evaluation and subsequent management.

Light’s Criteria, introduced in 1972, remain the most widely accepted and validated method for this purpose. They classify an effusion as exudative when there is evidence of increased capillary permeability or impaired lymphatic drainage, typically due to conditions like pneumonia, tuberculosis, pulmonary embolism, or cancer. In contrast, transudative effusions are usually linked to systemic processes such as heart failure, cirrhosis, or nephrotic syndrome, where fluid accumulates without direct pleural disease.

The criteria compare pleural fluid protein and LDH to serum values. If the ratios exceed certain thresholds, the effusion is considered exudative. This approach provides a sensitive method for detecting pleural pathology, though it may occasionally misclassify effusions, especially in patients on diuretics. In such cases, additional tools such as serum-effusion albumin gradient or clinical context are used.