If there is a buildup of fluid around the lungs (called a pleural effusion), doctors can perform thoracentesis to find out if it is caused by cancer spreading to the lining of the lungs (pleura). The buildup might also be caused by other conditions, such as heart or an infection.
For this procedure, the skin is numbed and a hollow needle is inserted between the ribs to drain the fluid.
In a similar test called pericardiocentesis, fluid is removed from within the sac around the heart.
The fluid is checked under a microscope for cancer cells.
Chemical tests of the fluid are also sometimes useful in telling a malignant (cancerous) pleural effusion from one that is not.