A 90-year-old man was admitted for progressive dyspnea. His medical history included hypertension and chronic myeloid leukemia diagnosed 10 years before and treated with dasatinib. Physical examination revealed pitting edema of the legs and dullness to percussion in the right chest. Laboratory test results were clinically relevant for white blood cells 7.7 ϫ 10 9 /L (reference interval 4 -10 ϫ 10 9 /L), platelets 162 ϫ 10 9 /L (150 -400 ϫ 10 9 /L), hemoglobin 10.5 g/dL (13-18 g/dL), total protein 7.1 g/dL After 12 h at 4°C, the pleural fluid had a white ring at the top of the sample (Fig. 1). DiscussionThe first step in evaluation of a pleural effusion is to differentiate exudative and transudative effusions; this plays a key role in determining the etiology of the effusion. Both exudative and transudative fluids are characterized by translocation of excess fluid in the pleural cavity. The main difference is the functional integrity of the pleural membranes. Transudative effusions are defined by unaltered pleural membranes and characterized by increased hydrostatic pressure or decreased oncotic pressure. Exudative effusions are characterized by altered pleural membranes associated with increased permeability.The main etiologic factors leading to a transudative pleural effusion are heart disease such as heart failure or pericarditis, cirrhosis, nephrotic syndrome, peritoneal dialysis, hypoalbuminemia, general disease such as sarcoidosis or hypothyroidism, and pulmonary embolism. Exudative pleural effusions are mainly caused by malignancy (e.g., mesothelioma, lung metastasis, lymphoma), infectious disease (e.g., tuberculosis, pyogenic infection, empyema), general disease (e.g., systemic lupus erythematous, sarcoidosis), pulmonary embolism, abdominal disease (e.g., hepatitis, pancreatitis, abdominal surgery), hemothorax, and chylothorax.Several biochemical assessments (total protein, LDH, glucose, albumin, and pH) have been proposed to discriminate between transudative and exudative pleural effusions. However, criteria defined by Light et al. in 1972 remain the gold standard (1 ). According to those criteria, a ratio of pleural fluid to plasma total protein of Ͼ0.5 is commonly found in exudative pleural effusions. Likewise, pleural fluid LDH activity Ͼ200 U/L and a ratio of pleural fluid to plasma LDH of Ͼ0.6 are consistent with an exudate. In our case, the ratio of pleural fluid to serum protein was 0.6 and LDH activity was 291 U/L, both suggesting an exudative effusion, whereas the ratio of pleural fluid to serum LDH was 0.57, close to the cutoff threshold.The diagnostic sensitivity of Light's criteria (1 ) for the diagnosis of exudative pleural effusion is close to 100%, but the specificity is lower, at 80% (2 ). Some transudative pleural effusions could be classified as exudative pleural effusions according to these criteria. The gradient between albumin concentration in pleural fluid and in the blood is commonly Ͻ1.2 g/dL in exudative pleural effusions (3 ).In our case, visual inspection of the pleural fluid o...