The analysis of pleural fluid (PF) is the most important diagnostic element in identifying the cause of pleural
effusions. The measurement of proteins and lactate dehydrogenase in PF and blood establishes the transudative or
exudative nature of effusions. Elevated concentrations of natriuretic peptide NT-proBNP (>1500 pg/mL) are virtually
pathognomonic of heart failure. In exudates with predominantly polymorphonuclear leukocytes (>50%), a bacterial
infection of the pleural space should be considered, particularly if pleural fluid C-reactive protein levels are high. A
pleural pH <7.20 in a parapneumonic effusion is an indication for the need of 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. The cytological study of PF is negative in 40% of
malignant effusions. Performing immunocytochemistry on the cell block and measuring tumor markers, such as
mesothelin, can increase the diagnostic yield for malignancy.
Keywords: Empyema, malignant pleural effusion, pleural effusion, tuberculous pleuritis, tumor markers.
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