How to diagnose Spontaneous Bacterial Peritonitis (SBP)?

From the Guidelines

Diagnosing spontaneous bacterial peritonitis (SBP) requires a diagnostic paracentesis with ascitic fluid analysis, and the diagnosis is made when the ascitic fluid absolute neutrophil count exceeds 250 cells/mm³, regardless of culture results. The procedure involves inserting a needle into the peritoneal cavity to withdraw fluid, which should be sent for cell count with differential, culture, and biochemical analysis 1. Additional tests should include ascitic fluid protein, albumin, glucose, and lactate dehydrogenase. The serum-ascites albumin gradient (SAAG) should be calculated to help determine the cause of ascites. Bedside inoculation of blood culture bottles with ascitic fluid increases the sensitivity of cultures. Clinical suspicion for SBP should be high in cirrhotic patients with ascites who develop fever, abdominal pain, altered mental status, or worsening liver or kidney function. Laboratory findings such as leukocytosis and elevated inflammatory markers may support the diagnosis but are not specific.

Key Points to Consider

• A diagnostic paracentesis should be carried out without delay to rule out SBP in all cirrhotic patients with ascites on hospital admission 1.
• The diagnosis of SBP is based on neutrophil count in ascitic fluid of >250/mm3, and neutrophil count is determined by microscopy or flow cytometry-based automated count 1.
• Ascitic fluid culture with bedside inoculation of blood culture bottles should be performed to guide the choice of antibiotic treatment when SBP is suspected 1.
• Patients with ascitic fluid PMN counts <250 cells/mm3 and signs or symptoms of infection should also receive empiric antibiotic therapy, while awaiting results of cultures 1.

Recommendations

• Diagnostic paracentesis should be performed in patients with GI bleeding, shock, fever or other signs of systemic inflammation, gastrointestinal symptoms, hepatic encephalopathy, and in patients with worsening liver or renal function 1.
• Ascitic fluid culture positivity is not a prerequisite for the diagnosis of SBP, but culture should be performed in order to guide antibiotic therapy 1.
• Empiric antibiotic therapy should be determined with due consideration of context of SBP, severity of infection, and local bacterial resistance profile 1.

From the Research

Diagnostic Criteria for SBP

• Spontaneous bacterial peritonitis (SBP) is diagnosed based on the analysis of ascitic fluid obtained by paracentesis 2, 3, 4, 5.
• An ascites absolute neutrophil count (ANC) ≥ 250 cells/mm3 is diagnostic of SBP 2, 3, 5.
• Ascitic fluid should be placed in blood culture bottles to improve the culture yield 2, 3.
• Leukocyte esterase reagent strips can be used for rapid diagnosis if available 2.

Clinical Presentation

• The typical presentation of SBP includes abdominal pain, worsening ascites, fever, or altered mental status in a patient with known liver disease 2.
• However, some patients may be asymptomatic or present with only mild symptoms 2.
• Abnormal ascitic fluid appearance has a high sensitivity for the detection of SBP, but clear fluid appearance does not completely rule out SBP 6.

Diagnostic Procedure

• Paracentesis is the diagnostic modality of choice and should be performed in any patient with ascites and concern for SBP or upper gastrointestinal bleeding, or in those being admitted for a complication of cirrhosis 2.
• Ultrasound should be used to optimize the procedure 2.
• Repeating the paracentesis after 48 hours of treatment to reculture the fluid and reassess the ascitic fluid neutrophil count appears to be the best way to assess efficacy of treatment 3.