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Free peritoneal fluid is common in animals with acute abdominal pain. Analysis of free peritoneal fluid is important for both determining the cause of acute abdomen and guiding the decision to take a patient to surgery. A FAST scan is very useful in identifying peritoneal fluid accumulations in small animal patients. Abdominocentesis (Video 3.1) can be performed with or without ultrasound guidance. Abdominal fluid samples should be examined for gross appearance, and analyzed for PCV, total solids, glucose, and cytology. Additional biochemical testing for creatinine and potassium, and bilirubin levels are also measured in cases of suspected uroperitoneum and bile peritonitis, respectively.

Video 3.1 Abdominocentesis in a cat.

The presence of hemoabdomen is confirmed by a PCV of peritoneal fluid similar to that of peripheral blood. This finding in a patient with no history of trauma and a normal coagulation profile is an indication for emergency laparotomy to identify and control the source of hemorrhage, often identified as a bleeding splenic or liver mass.

Septic peritonitis is confirmed by the cytologic presence of toxic neutrophils with intracellular bacteria and is also supported by an abdominal glucose measurement 20 mg/dl lower than that of the peripheral blood [8]. These findings are also an indication for emergency laparotomy to identify and control the source of sepsis, which is commonly a gastrointestinal perforation or rupture.

In cases of suspected bile peritonitis, intracellular bile pigment is found cytologically, and is noted to have a green or brown mucinous appearance [9]. In addition, if the total bilirubin measured in an abdominal fluid sample is greater than that in peripheral blood, bile peritonitis may be present. This also warrants emergency surgery following appropriate stabilization.

Uroperitoneum is confirmed by comparing levels of creatinine and potassium in abdominal fluid and serum. An abdominal fluid to serum ratio of creatinine greater than 2.0, and a fluid to serum ratio of potassium of greater than 1.4 (dogs) or 1.9 (cats) indicates uroperitoneum [10]. Following imaging studies to localize the source of uroperitoneum (retrograde urethrocystogram and, in patients with evidence of retroperitoneal pathology, excretory urography or pyelography), urinary diversion and/or emergency surgery is indicated.

Other types of abdominal fluid include pure transudates, proteinaceous effusions (modified transudates), neoplastic effusions, and chylous effusion. Patients with these types of effusions are less likely to require surgical intervention on an emergency basis.

Independent of abdominal fluid analysis, other indications for emergency exploratory laparotomy include any penetrating abdominal injuries such as from bite wounds or other trauma, the radiographic evidence of free gas in the peritoneum, small‐bowel obstruction, or gastric dilatation/volvulus syndrome.