When fluid builds up in the body—around the lungs (pleural), in the abdomen (ascitic), or around the heart (pericardial)—this test provides the definitive answer as to why. Your doctor collects a sample of this fluid, which our pathologists then examine under a microscope. They look for the cause, such as infections, inflammation, or abnormal (malignant) cells. This test is the most direct way to diagnose the cause of the fluid, providing the critical information needed for your treatment plan.