Malignant pleural effusion is a condition in which cancer causes an abnormal amount of fluid to collect between the thin layers of tissue (pleura) lining the outside of the lung and the wall of the chest cavity.

Clinical factors predicting the diagnosis of malignant pleural effusions are symptoms lasting more than 1 month and the absence of fever.

Biomarkers that have been shown to be raised in malignant pleural effusions compared to benign disease include vascular endothelial growth factor (VEGF), endostatin, matrix metalloproteinases and tumour markers such as carcinoembryonic antigen.

[10][11][12][13] Pleural fluid mesothelin has a sensitivity of 71%, greater than that of cytology, and a specificity of 89% for the diagnosis of malignant mesothelioma.

[16] However, this treatment requires an inpatient stay of approximately 2–7 days, can be painful and has a significant failure rate.