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PLEURAL effusions are commonly seen in a variety of diseases. Based on the underlying pathophysiological abnormality, these effusions can be divided into transudates and exudates.1 Transudative effusions develop when there is a change in systemic factors such as an increase in capillary hydrostatic pressure or a decrease in colloid osmotic pressure with no change in the pleural surface. Common causes of transudative effusions include congestive heart failure, chronic renal failure, and low protein states. Exudative effusions are caused by an altered permeability of the pleural vessels or by obstruction of lymphatics as is seen in neoplasms, infections, or other inflammatory processes.
Physicians use a number of laboratory tests to determine which of these processes is occurring in a particular patient. Light et al2 have shown that in a select group of patients, transudates and exudates can be separated in the laboratory by measuring the protein and lactate dehydrogenase
Peterman TA, Speicher CE. Evaluating Pleural Effusions: A Two-Stage Laboratory Approach. JAMA. 1984;252(8):1051–1053. doi:10.1001/jama.1984.03350080053027
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