An unusual cause of ascites is urinary ascites: this occurs when there is rupture of either the ureter or bladder, leading to leakage of urine into the peritoneal space. In adults, this is almost always due to either blunt trauma to the abdomen, or iatrogenesis, such as nicking the ureter during an abdominal surgery. While not a common diagnosis to make, urinary ascites should be considered after more usual causes of ascites, such as cirrhosis or nephrotic syndrome, have been excluded. An ascites fluid creatinine:serum creatinine ratio >1.0 is highly suggestive of an intraperitoneal urine leak. The peritoneal fluid is typically bland with few WBCs, unlike the situation in spontaneous bacterial peritonitis.
Creatinine is filtered some across the peritoneal membrane (in fact, this is actually used for versions of the peritoneal equilibration test, used in the titration of dialysis dosing for patients on PD), so it is normal to have some creatinine in the peritoneal fluid. But if the peritoneal creatinine is really high relative to the serum creatinine then it means the urine is draining directly into the peritoneal space. Some type of abdominal imaging (ultrasound, CT scan) is often helpful in terms of diagnosing where along the collecting system a rupture has occurred.