A patient with right-sided pleural effusion undergoes diagnostic thoracentesis. Light's criteria are applied. Which combination of findings classifies the effusion as an EXUDATE?
- A Pleural/serum protein ratio <0.5 AND pleural/serum LDH ratio <0.6 AND pleural LDH <2/3 upper limit of normal serum LDH
- B Pleural/serum protein ratio >0.5 OR pleural/serum LDH ratio >0.6 OR pleural LDH >2/3 upper limit of normal serum LDH ✓
- C Pleural fluid protein >3 g/dL alone, regardless of LDH
- D Pleural/serum protein ratio >0.3 AND pleural LDH >100 IU/L
Explanation
Light's criteria classify an effusion as an exudate if ANY ONE of three criteria is met: pleural fluid protein/serum protein ratio >0.5, pleural fluid LDH/serum LDH ratio >0.6, or pleural fluid LDH >2/3 of the upper normal limit for serum LDH. Meeting even one criterion classifies it as exudative. The criteria have ~98% sensitivity for exudates but ~75% specificity, occasionally misclassifying transudates (especially in diuretic-treated patients). The serum-effusion albumin gradient >1.2 g/dL identifies true transudates when Light's criteria incorrectly suggest an exudate.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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