The standard posteroanterior (PA) and lateral chest radiographs remain the most important techniques for the initial detection of pleural effusion.

A major role of ultrasonography in thoracentesis is to guide needles into small or loculated pleural effusions, thereby increasing both the yield and safety of the procedure.

The first diagnostic step in pleural effusion analysis is to classify the effusion as a transudate or an exudate by using the protein and lactate dehydrogenase values of serum and pleural fluid.

Transudative effusions are formed secondary to elevations in hydrostatic pressure or reductions in colloid osmotic pressure within the systemic or pulmonary circulation. Causes include the following:

Once a pleural fluid is established to be transudative in nature, additional pleural fluid studies are usually not indicated.

Once a pleural fluid is established to be exudative, all attempts possible must be made to determine the underlying cause.

Low pleural fluid glucose levels are associated with rheumatoid disease, TB, empyema, and certain tumors.

A high pleural fluid amylase can be seen in pancreatitis, pancreaticopleural fistulas, esophageal rupture, malignancy, and parapneumonic effusions.

A low pleural fluid pH (i.e., <7.30) may be seen with infected parapneumonic effusions, frank empyema, malignancy, collagen vascular disease, TB, esophageal rupture, and urinothorax.

Urinothorax is the only transudative effusion that can present with a low pH.

When ordering pH on pleural fluid, it is essential the fluid be aspirated anaerobically and transported on ice.

In large-volume thoracentesis, if a patient develops chest discomfort or an end-expiratory pleural pressure of less than -20 cm H₂0 by pleural manometry measurements, thoracentesis should be stopped. Continued fluid removal increases the risk of development of pneumothorax, edema in the underlying lung (reexpansion pulmonary edema), or rapid fluid shift from the intravascular space into the pleural space (postthoracentesis shock)