Diagnostic principles in pleural disease.

When a patient with an undiagnosed pleural effusion is evaluated, the first question to answer is whether the patient has a transudate or an exudate. This is best done using Light's criteria, but these criteria occasionally misidentify a transudate as an exudate. If the patient's pleural fluid meets exudative criteria, but the patient appears clinically to have a transudative effusion, then the serum-pleural fluid albumin gradient should be measured. If this is greater than 1.2 g-dL-1, the patient probably does have a transudative effusion. If the patient has an exudative pleural effusion, additional tests are indicated to determine the aetiology of the effusion. The gross appearance and the odour of the pleural fluid should be noted and samples of all exudates should be sent for bacterial cultures. Laboratory tests that are useful in the differential diagnosis of exudative pleural effusions include: differential white cell count of the pleural fluid; cytology of the pleural fluid; and levels of adenosine deaminase, glucose, amylase and lactate dehydrogenase in the pleural fluid. If pleural tuberculosis is suspected, a needle biopsy of the pleura is indicated. Thoracoscopy is very efficient at diagnosing malignant pleural effusion and tuberculosis pleuritis, but rarely establishes any other diagnosis.