Pleural Fluid LDH/Serum LDH Ratio >0.6 Favors Exudative Effusion
The correct answer is C: Pleural fluid LDH/serum LDH ratio >0.6, which is one of the three Light's criteria that definitively classifies an effusion as exudative. 1, 2
Light's Criteria for Exudative Effusions
An effusion is classified as exudative if it meets any one of the following three criteria: 1, 2
- Pleural fluid/serum protein ratio >0.5 (not <0.5 as in option A)
- Pleural fluid/serum LDH ratio >0.6 (this is option C - the correct answer)
- Pleural fluid LDH >67% of the upper limit of normal serum LDH
These criteria demonstrate approximately 98% sensitivity and 72% specificity for identifying exudative effusions, making them the gold standard for transudate-exudate differentiation. 1, 2
Why Each Option is Correct or Incorrect
Option A is incorrect: A pleural fluid protein/serum protein ratio <0.5** actually suggests a **transudate**, not an exudate. The threshold for exudate is **>0.5. 1
Option B is incorrect: This reverses the ratio. The correct criterion is pleural fluid LDH/serum LDH >0.6, not serum/pleural fluid LDH >0.6. 1
Option C is correct: Pleural fluid LDH/serum LDH ratio >0.6 is one of the three validated Light's criteria for exudative effusions. 1, 2
Option D is incorrect: pH >7.4 does not favor exudative effusions. In fact, low pH (<7.2) is associated with complicated parapneumonic effusions and empyema, which are exudates, but pH alone is not used to differentiate transudates from exudates. 2
Clinical Application
Meeting just one of the three Light's criteria thresholds is sufficient to classify the effusion as exudative; meeting none indicates a transudate. 1 Simultaneous collection of pleural fluid and serum samples is required to calculate these ratios accurately. 1
Common Pitfall
Light's criteria misclassify approximately 25-30% of cardiac or hepatic transudative effusions as exudates, especially in patients receiving diuretic therapy. 1 When clinical suspicion strongly suggests heart failure but Light's criteria indicate an exudate, use a serum-effusion albumin gradient >1.2 g/dL to reclassify approximately 80% of these false-positive exudates as transudates. 1, 3