The analysis of pleural fluid (PF) is the most important diagnostic element in identifying the cause of pleural effusions. PF biochemistries, in particular, are available immediately and offer relevant clinical information. The measurement of proteins and lactate dehydrogenase (LDH) in PF and blood (Light's criteria) establishes the transudative or exudative nature of effusions. Transudates are commonly caused by heart failure (HF), but diuretic therapy may concentrate PF causing higher levels of protein and/or LDH and, therefore, leading to a misclassification as an exudate. In this scenario, a serum-PF albumin gradient > 1.2 g/dL points to a true transudate. Furthermore, elevated concentrations of the natriuretic peptide N-terminal pro-brain natriuretic peptide are virtually pathognomonic of HF as the primary or, at least, secondary diagnosis. In exudates with predominantly polymorphonuclear leukocytes (> 50%), a bacterial infection of the pleural space should be considered, particularly if PF C-reactive protein levels are high. A pleural pH < 7.15 or a glucose < 40 mg/dL in a parapneumonic effusion indicates the need for a drainage tube. When lymphocytes predominate in an exudate, cancer and tuberculosis are the two main diagnoses to consider; an adenosine deaminase activity > 35 U/L strongly supports the diagnosis of tuberculosis. Measuring tumor markers (e.g., carcinoembryonic antigen, CA15-3) under the premise of using 100% specific cutoff points can increase the diagnostic yield for malignancy.

Introduction and objectives: Whether pleural effusions (PEs) secondary to pulmonary embolism can be exudative or transudative is controversial. This study aims to determine which type of effusion (exudate or transudate) is typically associated with pulmonary embolism, using Light's criteria as the discriminative gold standard. Methods: A retrospective analysis of all consecutive patients with pulmonary embolism subjected to a diagnostic thoracentesis over a 25-year period in a University Hospital was performed. Pleural fluid data were described in detail. Results: Seventy-one patients with pulmonary embolism-associated PEs comprised the study population. Pleural fluids were bloody in more than half the cases. The pleural fluid differential white blood cell count was variable; the predominant cells (> 50% of the total leukocytes) were lymphocytes in nearly two-thirds of the patients and neutrophils in about 30%. A proportion of eosinophils > 10% was observed in 7% of the cases. All fluids were exudates, meeting either 3 (78.2%), 2 (12.7%), or just 1 (9.1%) of the Light's criteria. Conclusion: The pleural fluid that accompanies pulmonary embolism is invariably an exudate.