<P> If malignant cells are present, a pathologist may perform additional studies including immunohistochemistry to determine the etiology of the malignancy . </P> <P> Chemistry studies may be performed including pH, pleural fluid: serum protein ratio, LDH ratio, specific gravity, cholesterol and bilirubin levels . These studies may help clarify the etiology of a pleural effusion (exudative vs transudative). Amylase may be elevated in pleural effusions related to gastric / esophageal perforations, pancreatitis or malignancy . Pleural effusions are classified as exudative (high protein) or transudative (low protein). </P> <P> In spite of all the diagnostic tests available today, many pleural effusions remain idiopathic in origin . If severe symptoms persist, more invasive techniques may be required . In spite of the lack of knowledge of the cause of the effusion, treatment may be required to relieve the most common symptom, dyspnea, as this can be quite disabling . Thoracoscopy has become the mainstay of invasive procedures as closed pleural biopsy has fallen into disuse . </P> <P> Diseases of the pleural cavity include: </P>

Describe the structure and function of the pleura