Light's criteria
Formal Definition
A set of three laboratory criteria used to classify pleural effusions as transudative (low protein, systemic cause) or exudative (high protein/LDH, local pleural pathology); an effusion is exudative if any one criterion is met: pleural protein/serum protein > 0.5, pleural LDH/serum LDH > 0.6, or pleural LDH > 2/3 the upper limit of normal serum LDH.
How It's Used on the Ward
"Send Light's criteria on the fluid" or "does it meet exudate criteria?" — the standard order set when a pleural tap is sent for analysis.
Example
""Thoracentesis sent: pleural protein 4.1 g/dL, serum protein 6.2 (ratio 0.66 > 0.5), pleural LDH 380, serum LDH 290 (ratio 1.31 > 0.6) — meets exudate criteria on both counts. Differential now includes malignancy, parapneumonic effusion, and PE.""
Clinical Context
Transudate: both protein and LDH ratios low — caused by CHF (most common), cirrhosis, nephrotic syndrome, hypoalbuminemia. Exudate: one or more criteria met — caused by parapneumonic effusion/empyema, malignancy, TB, PE, autoimmune (lupus, RA), viral pleuritis. High specificity for transudates, high sensitivity for exudates. Caveat: patients on diuretics may have "pseudo-exudate" — if clinical picture suggests CHF but fluid meets exudate criteria, check albumin gradient (serum-pleural albumin > 1.2 g/dL points to transudate). Additional exudate tests: glucose, pH, cytology, cultures, cell count, ADA (for TB).