Pleural effusion is usually diagnosed by a physical exam or a chest X-ray. In some cases, a CT scan may be used.
If pleural effusion is found, a doctor may perform a procedure called thoracentesis. During this procedure, fluid is collected from the pleural space for biochemical analysis, culture for infection, and assessment under a microscope for abnormal cells which may indicate cancer. This procedure is done to help find out why the fluid buildup is occurring. It can be done as an outpatient and is generally very safe.