According to Light's criteria, which finding indicates an exudative pleural effusion?

• A. Pleural fluid LDH to serum LDH ratio < 0.6
• B. Pleural fluid LDH less than two-thirds the ULN
• C. Pleural fluid protein to serum protein ratio > 0.5
• D. Pleural fluid protein to serum protein ratio < 0.5

Answer: (C)

Light's criteria distinguish exudative from transudative pleural effusions by comparing components of the pleural fluid to serum. An exudate arises from inflammation or malignant/infectious processes that increase capillary permeability, causing higher protein and LDH levels in the fluid.

The finding that pleural fluid protein relative to serum protein is greater than 0.5 is one of the defining rules. This elevated ratio indicates the fluid is not simply filtered plasma but contains more protein from inflammatory leakage, classifying the effusion as exudative.

Why the other statements aren’t the best fit: the LDH ratio criterion is actually that pleural fluid LDH to serum LDH ratio should be greater than 0.6, not less than 0.6. Another criterion is that pleural fluid LDH should be greater than two-thirds the upper limit of normal for serum LDH; stating it’s less than that threshold would not meet exudative criteria. Finally, a pleural fluid to serum protein ratio below 0.5 points toward a transudate, not an exudate.

So the statement of a pleural fluid protein to serum protein ratio greater than 0.5 best indicates an exudative pleural effusion.