Before a diagnosis of parapneumonic effusion can be made, a bacterial pneumonia needs to be present. Other possibilities for pulmonary infiltrates and pleural effusions need to be considered and may include pulmonary embolism, acute pancreatitis, Dressler's syndrome, and other diseases. With the involved side dependent, if the thickness exceeds 10 mm, a diagnostic thoracentesis should be performed immediately. An empyema or complicated pleural effusion can be identified only by examination of the pleural fluid.
Clinical evaluation is inadequate to predict simple sympathetic effusion from complicated effusion.
Pleural fluid should be examined grossly for color, turbidity, and odor. If pus is present, the pleural space should be drained via tube thoracostomy. In the absence of gross pus, pleural fluid is sent for other studies to assist in determining if immediate chest tube placement is required. These pleural fluid studies should include glucose, LDH, amylase, protein, pH, WBC count plus differential, Gram stain, aerobic and anaerobic bacterial cultures. Depending on the clinical setting, fungal and mycobacterial smears and cultures and cytological studies should be done.