After removal, the fluid may then be evaluated for:

Chemical composition including protein, lactate dehydrogenase (LDH), albumin, amylase, pH, and glucose
Gram stain and culture to identify possible bacterial infections
White and red blood cell counts and differential white blood cell counts
Cytopathology to identify cancer cells, but may also identify some infective organisms
Other tests as suggested by the clinical situation – lipids, fungal culture, viral culture, tuberculosis cultures, lupus cell prep, specific immunoglobulins

Lights criteria
Definitions of the terms "transudate" and "exudate" are the source of much confusion. Briefly, transudate is produced through pressure filtration without capillary injury while exudate is "inflammatory fluid" leaking between cells.Transudative pleural effusions are defined as effusions that are caused by systemic factors that alter the pleural equilibrium, or Starling forces. The components of the Starling forces – hydrostatic pressure, permeability, and oncotic pressure (effective pressure due to the composition of the pleural fluid and blood) – are altered in many diseases, e.g., left ventricular failure, kidney failure, liver failure, and cirrhosis. Exudative pleural effusions, by contrast, are caused by alterations in local factors that influence the formation and absorption of pleural fluid (e.g., bacterial pneumonia, cancer, pulmonary embolism, and viral infection).An accurate diagnosis of the cause of the effusion, transudate versus exudate, relies on a comparison of the chemistries in the pleural fluid to those in the blood, using Lights criteria. According to Lights criteria (Light, et al.
A polymerase chain reaction for tuberculous DNA may be done, or adenosine deaminase or interferon gamma levels may also be checked.The most common causes of exudative pleural effusions are bacterial pneumonia, cancer (with lung cancer, breast cancer, and lymphoma causing approximately 75% of all malignant pleural effusions), viral infection, and pulmonary embolism. Another common cause is after heart surgery when incompletely drained blood can lead to an inflammatory response that causes exudative pleural fluid. Conditions associated with exudative pleural effusions:
Parapneumonic effusion due to pneumonia
Malignancy (either lung cancer or metastases to the pleura from elsewhere)
Infection (empyema due to bacterial pneumonia)
Trauma
Pulmonary infarction
Pulmonary embolism
Autoimmune disorders
Pancreatitis
Ruptured esophagus (Boerhaaves syndrome)
Rheumatoid pleurisy
Drug-induced lupus

Other/ungrouped
Other causes of pleural effusion include tuberculosis (though stains of pleural fluid are only rarely positive for acid-fast bacilli, this is the most common cause of pleural effusions in some developing countries), autoimmune disease such as systemic lupus erythematosus, bleeding (often due to chest trauma), chylothorax (most commonly caused by trauma), and accidental infusion of fluids. Less common causes include esophageal rupture or pancreatic disease, intra-abdominal abscesses, rheumatoid arthritis, asbestos pleural effusion, mesothelioma, Meigss syndrome (ascites and pleural effusion due to a benign ovarian tumor), and ovarian hyperstimulation syndrome.Pleural effusions may also occur through medical or surgical interventions, including the use of medications (pleural fluid is usually eosinophilic), coronary artery bypass surgery, abdominal surgery, endoscopic variceal sclerotherapy, radiation therapy, liver or lung transplantation, insertion of ventricular shunt as a treatment method of hydrocephalus, and intra- or extravascular insertion of central lines.