Paracentesis is a procedure used to remove fluid that has collected in the stomach, called peritoneal fluid. This fluid buildup, called ascites, may be caused by infection, inflammation, injury, or other conditions, such as cirrhosis or cancer. The fluid is taken out using a long, thin needle inserted into the stomach. The fluid is then sent to a lab and studied to find the cause of the fluid buildup. Paracentesis is also used to remove fluid to relieve stomach pressure or pain in people with cancer or cirrhosis.