The differential diagnosis of pleural effusions.

The presence of pleural effusion enables a physician to obtain a specimen of a body cavity fluid easily. With a systematic analysis of the pleural fluid, in conjunction with the clinical features and ancillary laboratory data, a clinician should be able to arrive at either a presumptive or definitive diagnosis in approximately 90 percent of cases. Selectivity should be exercised in ordering analyses on pleural fluid. The first important deductive step is to decide whether the effusion is a transudate (due to imbalances in hydrostatic or oncotic pressures) or an exudate (inflammatory); serum protein and lactate dehydrogenase measurements will be decisive. The differential diagnosis of a transudate is relatively limited and usually easily discernible from the clinical presentation. The differential diagnosis of exudate poses a more difficult challenge for clinicians. The use of certain pleural fluid tests such as leukocyte count and differential, glucose, pH and, when indicated, pleural fluid amylase determinations, helps to narrow the differential diagnosis of an exudative pleural effusion.