A pleural effusion that changes from red to yellow often indicates that active bleeding has stopped for some time. The red color of pleural effusion generally indicates that part or all of the pleural fluid is blood; the change from red to yellow indicates that active bleeding has stopped, and the blood that has accumulated in the chest cavity, hemoglobin degradation into iron-containing hemoflavin, and then mixed with the fluid secreted by the pleura, which appears yellow. When pleural effusion turns yellow, it is also necessary to follow the doctor’s instructions for examination and treatment, and the cause of the disease needs to be clarified, and common causes include tumors in the chest cavity, tuberculosis infection, heart failure, etc. Secondly, for the pleural effusion itself, if it is a small amount, it is often waiting to be absorbed; if it is a large amount of medium or large amount, it is often necessary to pump or drain the fluid, especially for those who have chest tightness due to the compression of the lung tissue by the pleural effusion.