Paracentesis (from Greek κεντάω, "to pierce") is a form of body fluid sampling procedure, generally referring to peritoneocentesis (also called laparocentesis or abdominal paracentesis) in which the peritoneal cavity is punctured by a needle to sample peritoneal fluid.The procedure is used to remove fluid from the peritoneal cavity, particularly if this cannot be achieved with medication. The most common indication is ascites that has developed in people with cirrhosis. Indications
It is used for a number of reasons:
to relieve abdominal pressure from ascites
to diagnose spontaneous bacterial peritonitis and other infections (e.g. abdominal TB)
to diagnose metastatic cancer
to diagnose blood in peritoneal space in trauma

For ascites
The procedure is often performed in a doctors office or an outpatient clinic. In an experts hands, it is usually very safe, although there is a small risk of infection, excessive bleeding or perforating a loop of bowel. These last two risks can be minimized greatly with the use of ultrasound guidance.The patient is requested to urinate before the procedure; alternately, a Foley catheter is used to empty the bladder. The patient is positioned in the bed with the head elevated at 45–60 degrees to allow fluid to accumulate in lower abdomen. After cleaning the side of the abdomen with an antiseptic solution, the physician numbs a small area of skin and inserts a large-bore needle with a plastic sheath 2 to 5 cm (1 to 2 in) in length to reach the peritoneal (ascitic) fluid. The needle is removed, leaving the plastic sheath to allow drainage of the fluid.
A count of 250 neutrophils per ml or higher is considered diagnostic for spontaneous bacterial peritonitis. Cultures of the fluid can be taken, but the yield is approximately 40% (72-90% if blood culture bottles are used). Contraindications
Mild hematologic abnormalities do not increase the risk of bleeding. The risk of bleeding may be increased if:
prothrombin time > 21 seconds
international normalized ratio >  1.6
platelet count < 50,000 per cubic millimeter.Absolute contraindication is the acute abdomen that requires surgery. Relative contraindications are:
Pregnancy
Distended urinary bladder
Abdominal wall cellulitis
Distended bowel
Intra-abdominal adhesions. See also
Anterior chamber paracentesis

References
External links

WebMD: Patient guide