Light's Criteria Calculator — Pleural Exudate vs Transudate

Formula

How this is calculated

Light's criteria were described by Richard Light in 1972 and remain the reference standard for pleural effusion classification. An effusion is classified as an EXUDATE if at least one of three biochemical tests is met: the pleural fluid-to-serum protein ratio exceeds 0.5, the pleural fluid-to-serum LDH ratio exceeds 0.6, or the absolute pleural fluid LDH exceeds two-thirds of the laboratory's upper limit of normal (ULN) for serum LDH. If none is met, the effusion is a TRANSUDATE.

Transudates arise from hydrostatic or oncotic pressure imbalances — classically congestive heart failure, hepatic cirrhosis, or nephrotic syndrome — and rarely require extensive investigation of the pleural space. Exudates result from pleural inflammation or disrupted capillary permeability, and their causes (pneumonia, malignancy, pulmonary embolism, tuberculosis, and others) require further workup.

Light's criteria have sensitivity of 97–100% for exudate, but specificity of only about 74–83%. Approximately 20–30% of transudates due to heart failure on diuretic therapy are misclassified as exudates. In that setting, the serum–effusion albumin gradient (SEAG = serum albumin − pleural albumin) of ≥ 12 g/L (1.2 g/dL) better identifies true transudates. Always correlate with the clinical picture, imaging, and additional tests (cell count, glucose, pH, cytology, culture) as indicated.