A 65-year-old man presents with left-sided pleuritic chest pain and breathlessness. A pleural tap shows: straw-coloured fluid, LDH 350 IU/L (serum LDH 280 IU/L), protein 3.8 g/dL (serum protein 6.5 g/dL). Which criterion best classifies this as an exudate by Light's criteria?

• A Pleural fluid LDH >2/3 of upper limit of normal for serum LDH
• B Pleural fluid protein >3.0 g/dL (absolute value)
• C Pleural fluid LDH to serum LDH ratio >0.6
• D Pleural fluid protein to serum protein ratio >0.5 ✓

Explanation

Light's criteria classify pleural fluid as an exudate if any ONE of the following three is met: (1) pleural/serum protein ratio >0.5; (2) pleural/serum LDH ratio >0.6; (3) pleural LDH >2/3 of upper limit of normal for serum LDH. In this patient: protein ratio = 3.8/6.5 = 0.58 (>0.5 = exudate), and LDH ratio = 350/280 = 1.25 (>0.6 = exudate). Absolute protein >3.0 g/dL is a historical criterion but NOT part of Light's formal criteria — it's less reliable. Light's criteria correctly classify >97% of exudates and transudates.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.