Pleural fluid can establish a definitive diagnosis in a limited number of diseases, such as empyema, malignancy, chylothorax, and rheumatoid pleurisy. However, it is highly useful in excluding potentially harmful diseases that would warrant immediate intervention, such as empyema.

Initial evaluation of the fluid is performed at the time of thoracentesis, as the fluid is aspirated. Careful attention should be paid to the color (straw colored, serosanguinous, bloody, white), consistency (pus, turbid, debris), and odor (foul smelling) of the fluid.

After visual inspection during the procedure, the fluid is sent for laboratory analysis. Broad classification of the fluid into transudative or exudative by chemical analysis is performed (Tables 56.3 and 56.4). Richard Light established a well-known algorithm for distinguishing an exudative pleural effusion based on three tests: (a) pleural fluid lactate dehydrogenase (LDH) >two-thirds the laboratory’s upper limit of normal for serum, (b) pleural fluid to serum LDH ratio >0.6, and (c) pleural fluid to serum protein ratio >0.5. Only one of these results needs to be positive to confirm an exudative effusion. Light’s criteria has a diagnostic accuracy over 90 % but drops significantly to below 70–80 % if one of the three categories is borderline. Pleural fluid can also be analyzed for a number of other laboratory tests, including but not exclusive, to glucose, pH, amylase, cholesterol, albumin, B-type natriuretic peptide (BNP), and adenosine deaminase (ADA). There are many other tests and ways to analyze the pleural fluid from a thoracentesis, but that is outside the scope of this chapter.