Is it meaningful to use biochemical parameters to discriminate between transudative and exudative pleural effusions?

OBJECTIVES: The usefulness of biochemical criteria to separate pleural transudates from exudates is controversial, and the limitations of Light's criteria in patients receiving diuretic therapy is of general concern. We evaluated the added value of biochemical criteria to clinical judgment for separating transudates from exudates. SETTING: A community teaching hospital. DESIGN: A prospective, observational study for the evaluation of diagnostic tests. PATIENTS AND MEASUREMENTS: In 249 consecutive patients referred for diagnostic thoracentesis, two physicians classified the pleural effusion as transudate or exudate based on all available information just before performing the thoracentesis. The sensitivity, specificity, and accuracy of the clinical presumption were compared with those of Light's criteria, and serum-pleural fluid albumin and protein gradients. The combined accuracy of biochemical and clinical criteria was also assessed. RESULTS: The accuracy of Light's criteria (93%) was significantly higher than that showed by the initial clinical presumption (84%; p < 0.01) and that of the alternative biochemical criteria: serum-pleural fluid albumin gradient (87%; p < 0.03) and serum-pleural fluid protein gradient (86%; p < 0.01). In patients receiving diuretic therapy, the accuracy of Light's criteria was 83% (60 of 72 cases), neither different to that of the albumin gradient (88%; 63 of 72 cases) nor to that of the protein gradient (86%; 62 of 72 cases). When these alternative biochemical criteria were applied exclusively to patients receiving diuretics who were thought to have a transudative effusion by clinical criteria, but having fluid identified as exudative by Light's criteria, the results did not improve significantly. Using a multiparametric model for the population receiving diuretics, the accuracy was greater (93%; 67 of 72 cases) than that of Light's criteria but without reaching statistical significance (p = 0.12). CONCLUSIONS: Light's criteria are significantly superior to the clinical presumption to separate pleural transudates from exudates. In patients receiving diuretics, Light's criteria lose accuracy, which is similar to that showed by the use of alternative biochemical criteria alone or combined with clinical judgment.