THORACENTESIS AND PLEURAL FLUID ANALYSIS Thoracentesis may be performed for diagnosis or therapy. Thoracentesis is diagnostic in approximately 75% of patients; even when not diagnostic, it helps exclude other important diagnoses such as empyema. Diagnostic thoracentesis requires a relatively small amount of material (30 to 50 mL). As a rule, newly discovered effusions should be tapped. Although there are no absolute contraindications to diagnostic thoracentesis, relative contraindications include a bleeding diathesis, anticoagulation, a small volume, mechanical ventilation, and a low benefit-to-risk ratio. Therapeutic thoracentesis involves removing larger amounts of fluid (no more than 1000 to 1500 mL at one time because edema may occur in the re-expanded underlying lung, especially in cases of tension effusion). Transudative and exudative pleural effusions are distinguished by measuring the lactate dehydrogenase (LDH) and protein levels in the pleural fluid. Exudative pleural effusions meet at least one of the following criteria, whereas transudative pleural effusions meet none: 1.Pleural fluid protein/serum protein >0.5 2.Pleural fluid LDH/serum LDH >0.6 3.Pleural fluid LDH more than two-thirds normal upper limit for serum These criteria misidentify ~25% of transudates as exudates. If one or more of the exudative criteria are met and the patient is clinically thought to have a condition producing a transudative effusion, the difference between the protein levels in the serum and the pleural fluid should be measured. If this gradient is >31 g/L (3.1 g/dL), the exudative categorization by these criteria can be ignored because almost all such patients have a transudative pleural effusion.