Light's Criteria for Pleural Effusion — SCE Palliative Medicine MCQ
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Correct answer: B — Pleural fluid protein/serum protein ratio >0.5, OR pleural fluid LDH/serum LDH ratio >0.6, OR pleural fluid LDH >2/3 the upper limit of normal serum LDH
Explanation lettering: B = shown as A · E = shown as B · D = shown as C · C = shown as D · A = shown as E
By Light’s criteria, a pleural effusion is classified as exudative if it meets any one of three biochemical thresholds: pleural fluid/serum protein ratio >0.5; pleural fluid/serum LDH ratio >0.6; or pleural fluid LDH greater than two‑thirds of the upper limit of normal for serum LDH. Option E succinctly and accurately states this. Other options are plausible red herrings: pH (A) and glucose (B) are relevant to complicated parapneumonic effusions or malignancy but are not part of Light’s diagnostic criteria; absolute low protein (C) is incorrect, as exudates have higher, not lower, protein; straw colour (D) is nonspecific.
Reference: Pleural fluid biochemistry – UH Sussex (updated May 19 2026) & Barts Health NHS Trust Pleural Disease Guideline, 2025