Pleural Effusions: The Diagnostic Separation of Transudates and Exudates

In this prospective study of 150 pleural effusions, the utility of pleural-fluid cell counts, protein levels, and lactic dehydrogenase (LDH) levels for the separation of transudates from exudates was evaluated. According to preset diagnostic criteria, 47 of the effusions were classified as transudates and 103 as exudates. Three characteristics were found, each of which was associated with over 70% of the exudates and, at most, one of the transudates: 1 a pleural fluid-to-serum protein ratio greater than 0.5; 2 a pleural fluid LDH greater than 200 IU; and 3 a pleural fluid-to-serum LDH ratio greater than 0.6. Moreover, all but one exudate had at least one of these three characteristics, whereas only one transudate had any of the three. The simultaneous use of both the pleural-fluid protein and LDH levels better differentiates transudates from exudates than does the use of either of these values individually.