Light's Criteria
Pleural Effusion — Exudate vs. Transudate
Any ONE criterion met → exudate. None met → transudate.
For the FNP boards, the question after a thoracentesis is always exudate or transudate, and Light's criteria answer it with three ratios. Fluid is exudative if any one of: pleural protein to serum protein ratio over 0.5, pleural LDH to serum LDH ratio over 0.6, or pleural LDH over two-thirds of the upper limit of normal serum LDH. Transudates (CHF, cirrhosis, nephrotic syndrome) need none. Exudates (parapneumonic, malignancy, TB, PE) need at least one. AANP exam vignettes test the protein or LDH cutoff and the next step — empiric diuresis for transudates, more workup or drainage for exudates.
- 1Pleural protein / serum protein >0.5Send pleural fluid AND serum simultaneously — ratios fail if timing diverges.
- 2Pleural LDH / serum LDH >0.6Same sampling rule as protein.
- 3Pleural LDH > 2/3 upper limit of normal serum LDHUses the lab's upper reference range, not the patient's own serum LDH.
Clinical Context
Separates exudate (inflammation, infection, malignancy) from transudate (hydrostatic/oncotic — CHF, cirrhosis, nephrotic syndrome). 98% sensitive for exudate; trades some specificity (will misclassify ~25% of CHF-related transudates as exudate if patient has been on diuretics).
Typical transudate causes: CHF (most common), cirrhosis, nephrotic syndrome, atelectasis, peritoneal dialysis. Typical exudate causes: parapneumonic effusion, malignancy, PE, TB, connective-tissue disease, chylothorax.
If Light's calls it exudate but clinically you strongly suspect CHF on diuretics, check serum-to-pleural albumin gradient >1.2 g/dL — that reclassifies to transudate. AANP tests Light's directly and pairs it with a clinical picture (dyspnea + lower extremity edema + effusion) to build the DDx.
Related Mnemonics
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