Prospective study to determine the volume of pleural fluid required to diagnose malignancy.

Background: The optimal volume of pleural fluid to diagnose a malignant effusion is unknown. Our study was designed to demonstrate if a minimum pleural fluid volume (10 mL) is equivalent to a large volume thoracentesis to make a cytopathologic diagnosis of malignancy.

Methods: A total of 121 thoracentesis samples were obtained from 102 patients with suspected or known malignant effusions.