Traditionally, Lights criteria have been used to differentiate transudative from exudative effusions. These criteria include: (1) pleural fluid protein to serum protein ratio greater than 0.5, (2) pleural fluid LDH to serum LDH ratio greater than 0.6, and (3) pleural fluid LDH ratio greater than two thirds the upper limit of normal for the serum LDH. Exudative effusions meet at least 1 of Lights criteria, and transudates meet none. Recently, several other tests have been proposed as well. A pleural fluid cholesterol concentration greater than 60 mg/dL and a pleural fluid to serum albumin gradient of less than 1.2 may also suggest an exudative effusion.2 Pleural fluid glucose, pH, and cell count levels are helpful in characterizing pleural effusions but do not differentiate between transudates and exudates.
2. Light RW. Pleural diseases. 4th ed. Philadelphia: Lippincott Williams & Wilkins; 2001:44-45, 57-8, 163-5.
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