Pleural Effusion (Light's Criteria) — Pathway

Step-by-step decision

Choose step by step as prompted; reaching an endpoint gives the management recommendation. You can go back a step or restart anytime.

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Full pathway

1. [Decision] Light's criteria classification
   After thoracentesis, Light's criteria — exudate or transudate? (Diagnostic thoracentesis, send pleural fluid + serum protein and LDH (and cell count/differential, glucose, pH, culture, cytology, ADA-TB, triglycerides as needed). Light's criteria (any one met = exudate): 1) pleural/serum protein >0.5; 2) pleural/serum LDH >0.6; 3) pleural LDH >2/3 the upper limit of normal serum LDH. Suspected hemothorax/empyema → emergency chest drainage (before classification).)
   • Any met (exudate) → Exudate · find the cause
   • None met (transudate) → Transudate
2. [End] Transudate
   Transudate (no Light's criteria met): usually heart failure, cirrhosis, nephrotic syndrome — treat the underlying disease, diurese. Note ~15–25% of transudates (especially CHF on diuretics) are misclassified as exudates; a serum-pleural albumin gradient >1.2 g/dL can correct them to transudate.
3. [End] Exudate · find the cause
   Exudate (any Light's criterion met): further work-up (cytology, culture, ADA-TB, biopsy if needed) — common causes parapneumonic/malignant/tuberculous (common in some regions)/pulmonary embolism/empyema. Complicated parapneumonic/empyema (pH <7.2, low glucose, pus, positive culture, loculation) → chest drainage ± fibrinolytics/surgery; large symptomatic → therapeutic thoracentesis.

Source guidelines & references

• Light's criteria (Light RW, 1972); AAFP diagnosis of pleural effusion

This pathway is our own synthesis of the decision logic in the guidelines above (not the guideline verbatim); thresholds and workflows change as guidelines update — in practice follow the latest guideline, your institution's protocol and the individual patient.

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