Peritoneal fluid is a liquid that is made in the abdominal cavity to lubricate the surface of the tissue that lines the abdominal wall and pelvic cavity and covers most of the organs in the abdomen. An increased volume of peritoneal fluid is called ascites.
Sampling of peritoneal fluid is generally performed by peritoneocentesis.
Peritoneal Fluid Analysis 
The serum-ascites albumin gradient (SAAG) is the most useful index for evaluating peritoneal fluid and can help distinguish ascites caused by portal hypertension (cirrhosis, portal vein thrombosis, Budd-Chiari syndrome, etc) from other causes of ascites. SAAG is calculated by subtracting the albumin measure of ascitic fluid from the serum value. In portal hypertension, the SAAG is >1.1 g/dL while ascites from other causes shows a SAAG of less than 1.1 g/dL.
Peritoneal fluid microscopy is a useful test in evaluating the cause of ascites. A diagnostic peritoneal lavage (DPL) is considered positive if any of the following are present  1. >15 mL gross blood 2. RBCs >100,000/mL 3. WBCs >500/mL 4. Bacteria present on Gram Stain
- De Mais, Daniel. ASCP Quick Compendium of Clinical Pathology, 2nd Ed. ASCP Press, Chicago, 2009.
- Peritoneal fluid entry in the public domain NCI Dictionary of Cancer Terms
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