Light's criteria are used to classify pleural effusion as exudative. A 58-year-old patient post-CABG day 4 has a left pleural effusion. Pleural fluid analysis: protein 3.8 g/dL, LDH 180 IU/L, serum protein 6.8 g/dL, serum LDH 420 IU/L. By Light's criteria, this is an exudate. However, the clinical picture suggests a transudative process (post-cardiac surgery). Which test would most reliably RECLASSIFY this as a transudate?
- A Pleural fluid serum albumin gradient (PFAG) > 1.2 g/dL indicates a transudate ✓
- B Pleural fluid cholesterol < 55 mg/dL
- C Pleural fluid NT-proBNP > 1500 pg/mL
- D Pleural fluid glucose < 60 mg/dL
Explanation
Light's criteria have a high sensitivity (~98%) but lower specificity (~75%) for exudates, resulting in 'pseudoexudates' — truly transudative effusions that meet exudate criteria by Light's, typically in patients on diuretics or with post-cardiac surgery effusions where serum proteins are diluted. In these cases, a serum-pleural albumin gradient (SPAG) of > 1.2 g/dL indicates a transudate, effectively reclassifying the pseudoexudate. This is the recommended supplementary test. Pleural NT-proBNP > 1500 pg/mL supports cardiac failure but does not reclassify by Light's system. Cholesterol < 55 mg/dL is an alternative criterion for transudate but is not as standardised. Glucose < 60 mg/dL suggests complicated parapneumonic effusion, rheumatoid arthritis, or malignancy.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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