Light's criteria are used to differentiate exudative from transudative pleural effusions. Which combination of findings is diagnostic of an EXUDATE?
- A Pleural fluid LDH/serum LDH ratio >0.6 ✓
- B Pleural fluid protein <3 g/dL
- C Pleural fluid/serum protein ratio <0.5
- D Serum albumin minus pleural albumin gradient >1.2 g/dL
Correct answer: A. Pleural fluid LDH/serum LDH ratio >0.6
Explanation
Light's criteria (1972) define an exudate if any ONE of three criteria is met: (1) pleural fluid protein/serum protein ratio >0.5, (2) pleural fluid LDH/serum LDH ratio >0.6, or (3) pleural fluid LDH >2/3 upper normal limit of serum LDH. A pleural fluid protein <3 g/dL and protein ratio <0.5 suggest a transudate. The serum-effusion albumin gradient >1.2 g/dL is used to reclassify heart failure effusions mislabelled exudates by Light's criteria (they have gradient >1.2 despite meeting exudative criteria).
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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