Various methods can be used to classify pleural fluid. By the origin of the fluid: * Serous fluid (hydrothorax) * Blood (haemothorax) * Chyle (chylothorax) * Pus (pyothorax or empyema) * Urine (urinothorax) By pathophysiology: * Transudative<unk>  * Exudative<unk>  By the underlying cause (see next section).
Pleural effusion
Pleural empyema
Pleurisy
Parapneumonic effusion
Hydrothorax
Pneumothorax
Malignant pleural effusion
Hemothorax