Small pleural effusions are most easily detected on a lateral decubitus film with the affected side down. Accumulations as little as 5 to 50 mL of fluid can be detected with this view.
“Contrast-enhanced CT can help differentiate benign and malignant disease in patients with an exudative pleural effusion, although the specific findings are not always seen (SOR C). PET seems promising for differentiating benign and malignant pleural disease. Aspiration should not be performed in patients with bilateral effusions if the clinical findings strongly suggest a pleural transudate, unless there are atypical features or the effusion fails to respond to therapy (SOR C). About 20% of patients with a pleural effusion caused by heart failure may fulfill the criteria for an exudative effusion after receiving diuretics. In these cases, if the difference between the protein levels in the serum and the pleural fluid is >3.1 g/dL, the patient should be classified as having a transudative effusion (SOR C).
Lymphocytosis in pleural fluid suggests tuberculosis or lymphoma, and can also suggest a post-CABG effusion, although the lymphocyte count will not be as high (SOR B). Cytology is positive in approximately 60% of malignant pleural effusions (SOR B). The diagnostic yield may be improved by additional pleural taps. If malignancy is still a concern, thoracoscopy should be considered (SOR C).”
http://www.aafp.org/afp/2006/0401/p1211.html
http://thorax.bmj.com/content/65/Suppl_2/ii4.long