Introduction: Spontaneous bacterial peritonitis (SBP) occurs in up to 30% of patients with cirrhosis and has an estimated in-hospital mortality rate of 20%. The prevalence of SBP in cirrhotic outpatients is 1.5 to 3.5% and among inpatients is approximately 10%. In most instances, SBP results from translocation of bacteria from the intestinal lumen. Less often, SBP results from bacteremia that originates at a distant site, such as a urinary tract infection. Most cases of SBP are caused by gram-negative enteric organisms, such as Escherichia coli and Klebsiella pneumoniae. Risk factors for the development of SBP include ascitic fluid total protein less than 1 g/dL, gastrointestinal hemorrhage, and previous history of SBP.
Indications for Testing: In a patient with ascites, the presence of new onset fever (temperature greater than 37.8°C or 100°F), abdominal pain, hepatic encephalopathy, metabolic acidosis, renal failure, hypotension, diarrhea, paralytic ileus, hypothermia, leukocytosis, or other sign or symptom of infection should prompt a diagnostic paracentesis for ascitic fluid analysis and culture (Figure 1). Approximately 13% of patients with SBP can present without any symptoms. Because SBP is so commonly present at the time of any type of admission for a cirrhotic patient, a diagnostic paracentesis is recommended routinely for these patients at the time of admission. There is no need for transfusion of plasma or platelets prior to a diagnostic paracentesis, given the extremely low risk of hemorrhagic complications, except in the setting of disseminated intravascular coagulation or clinically apparent fibrinolysis.
Diagnostic Criteria: The diagnosis of confirmed spontaneous bacterial peritonitis requires an elevated ascitic fluid absolute polymorphonuclear leukocyte (PMN) count of at least 250 cells/mm3 (0.25 x 109/L) and a positive ascitic fluid bacterial culture without an obvious intra-abdominal source of infection. Ascitic fluid diagnostic testing should be performed before treatment is initiated as even a single dose of broad-spectrum antibiotics can lead to no growth on bacterial culture in 86% of cases. Approximately 1 mL of ascitic fluid should be injected into a “purple-top” EDTA tube for the cell count and differential analysis. In the case of a traumatic paracentesis, with the entry of blood into the ascitic fluid (typically ascitic red cells greater than 10,000 cells/mm3) the PMN count should be corrected by subtracting one PMN for every 250 red cells/mm3 from the absolute PMN count.
Bacterial Culture: Prior to administering antibiotics, ascitic fluid (at least 10 mL) should be inoculated directly into a blood culture bottle at the bedside, instead of sending the fluid to the laboratory in a syringe or container, since immediate inoculation improves the yield on bacterial culture from approximately 65% to 90%, when the ascitic fluid cell count is at least 250 cells/mm3 (0.25 x 109/L). Separate and simultaneous blood cultures should also be obtained, as up to 50% of patients with SBP have concomitant bacteremia.
Importance of Distinguishing SBP from Secondary Bacterial Peritonitis: It is important to distinguish SBP from secondary bacterial peritonitis because of the critical need to determine whether surgical intervention is needed. Specifically, mortality approaches 100% in patients with secondary bacterial peritonitis who receive treatment with antibiotics alone (without surgery) and mortality is approximately 80% in patients with cirrhosis and SBP who undergo an unnecessary exploratory laparotomy.
Diagnostic Tests for Distinguishing SBP from Secondary Bacterial Peritonitis: Diagnostic tests may help distinguish SBP from secondary bacterial peritonitis due to a perforated viscus or a loculated abscess. Characteristically, with secondary bacterial peritonitis, the fluid PMN count is at least 250 cells/mm3 (usually greater than several thousand) and multiple organisms, including fungi, are identified on Gram’s stain and isolated in culture. Laboratory diagnostic criteria for secondary bacterial peritonitis includes two of the following: ascitic fluid protein greater than 1 g/dL, lactate dehydrogenase higher than the upper limit of normal for serum, and glucose less than 50 mg/dL. In addition, ascitic fluid carcinoembryonic antigen greater than 5 ng/mL and alkaline phosphatase greater than 240 U/L have been shown to be associated with gut perforation. After 48 hours of appropriate antibiotic therapy, the ascitic fluid PMN count should decrease with SBP (typically below the pre-treatment level), but with secondary bacterial peritonitis the PMN count may increase. Patients who meet criteria for secondary bacterial peritonitis should undergo immediate abdominal imaging, and emergent laparotomy should be considered if perforation or a surgically treatable site of infection is identified or strongly suspected.
Other Diagnostic Tests on Ascitic Fluid: For an initial diagnostic paracentesis, other tests should be performed as clinically warranted on the remaining ascitic fluid. These tests can be submitted to the laboratory using a “red-top” tube and may include albumin, total protein, glucose, lactate dehydrogenase, amylase, and bilirubin. A serum-ascites gradient (SAAG) of 1.1 g/dL or greater is consistent with portal hypertension. A total protein level of less than 1.0 g/dL is associated with an increased risk of spontaneous bacterial peritonitis. Elevated total protein, low glucose concentration, and elevated lactate dehydrogenase ascitic values are seen in the setting of secondary bacterial peritonitis. Elevated ascitic amylase can be seen in pancreatitis and gut perforation. Biliary leakage into the peritoneum can be associated with increased ascitic fluid bilirubin concentration. For patients with a prior paracentesis, especially a recent paracenteis, most of these additional diagnostic tests will not need repeating.