In clinical practice, it remains a challenge to distinguish between malignant pleural effusion (MPE) and benign pleural effusion (BPE). Because of the limitations of conventional cytology methods, there is an urgent need to develop a more accurate and effective method for the efficient diagnosis of MPE. In collaboration with the University of Arizona, Precision Epigenomics has developed the EPISEEK-MPE test that has the potential to be an effective complimentary tool for cytology in the process of differential diagnosis. The aim of this project is to conduct a multi-site clinical validation study to establish the EPISEEK-MPE test as a new commercial molecular test for the diagnosis of malignant pleural effusions. The study will: 1) Establish the diagnostic value of the EPISEEK\_MPE liquid biopsy assay to identify cancer in pleural effusion and 2) Validate the EPISEEK-MPE liquid biopsy assay for cancer detection in pleural effusion. The long-term goal of this project is to bring the field of pathology a new diagnostic tool for the diagnosis of malignant pleural effusions.