Useful tests on the pleural fluid in the management of patients with pleural effusions

Examination of the pleural fluid is useful in establishing the etiology of a pleural effusion. Transudative pleural effusions can be differentiated from exudative pleural effusions by measuring the levels of protein and lactic acid dehydrogenase in the pleural fluid and serum. If a patient clinically appears to have a transudative pleural effusion, but the pleural fluid meets exudative criteria, demonstration that the albumin levels is more than 1.2 gm/dl higher in the serum than in the pleural fluid provides evidence that the effusion is transudative. The gross appearance of the pleural fluid should always be noted. Other tests that routinely should be obtained on exudative pleural fluids are Gram stain and cultures, cell counts and differential, glucose, amylase, lactic acid dehydrogenase, cytology, and a marker for tuberculous pleuritis. The diagnosis of tuberculous pleuritis is strongly suggested by a pleural fluid adenosine deaminase level above 45 IU/L or a gamma interferon level above 3.7 U/ml.