A 30-year-old man presents with fever, cough, and right-sided pleuritic chest pain. Chest X-ray shows right pleural effusion. Diagnostic thoracentesis: straw-colored fluid, glucose 45 mg/dL, LDH 500 IU/L (serum LDH 250), protein 5 g/dL (serum protein 7 g/dL). What is the MOST likely cause?

• A Transudative effusion — congestive heart failure
• B Chylothorax
• C Hemothorax
• D Exudative effusion — parapneumonic or tuberculous pleuritis ✓

Explanation

Light's criteria classify exudates when at least one criterion is met: pleural/serum protein ratio > 0.5, pleural/serum LDH ratio > 0.6, or pleural LDH > 2/3 upper limit of serum normal. Here, pleural/serum protein = 5/7 = 0.71 (>0.5) and pleural/serum LDH = 500/250 = 2.0 (>0.6)—both criteria are met, confirming an exudate. Low pleural glucose (45 mg/dL) further supports TB pleuritis or complicated parapneumonic effusion. Heart failure causes transudates. Chylothorax has milky fluid with elevated triglycerides; hemothorax has bloody fluid.

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

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Written and medically reviewed by the StethoPrep medical team.