A 55-year-old man with chronic kidney disease on hemodialysis presents with dyspnea and bilateral pleural effusions. Thoracentesis shows pleural fluid protein 2.0 g/dL, serum protein 5.5 g/dL, pleural fluid LDH 100 U/L, serum LDH 250 U/L (upper limit of normal 250 U/L). According to Light's criteria, this effusion is best classified as:

• A Transudate, because none of Light's criteria are met ✓
• B Exudate, because pleural fluid LDH exceeds two-thirds the upper limit of normal serum LDH
• C Exudate, because pleural fluid protein exceeds 2.5 g/dL
• D Exudate, because the pleural fluid-to-serum LDH ratio exceeds 0.6

Explanation

Light's criteria classify an effusion as exudative if any one of the following is met: pleural fluid-to-serum protein ratio greater than 0.5, pleural fluid-to-serum LDH ratio greater than 0.6, or pleural fluid LDH greater than two-thirds the upper limit of normal serum LDH. Here, the protein ratio is 0.36, LDH ratio is 0.40, and pleural LDH (100) is below two-thirds of the upper limit (167). None of the criteria are met, confirming a transudate. The most tempting distractor is option B, but 100 U/L does not exceed two-thirds of 250 U/L.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.