Diagnostic Criteria for Spontaneous Bacterial Peritonitis
The diagnosis of SBP is confirmed when ascitic fluid polymorphonuclear (PMN) leukocyte count is >250 cells/mm³, regardless of culture results. 1, 2
When to Perform Diagnostic Paracentesis
Perform diagnostic paracentesis immediately in all cirrhotic patients with ascites upon hospital admission, even without symptoms of infection. 1, 2 This is mandatory because up to one-third of SBP patients are completely asymptomatic or present only with encephalopathy or acute kidney injury. 1
High-Risk Clinical Scenarios Requiring Urgent Paracentesis
Perform diagnostic paracentesis emergently in patients with any of the following: 1, 2
- Fever, hypothermia, or signs of systemic inflammation
- Abdominal pain or gastrointestinal symptoms
- Hepatic encephalopathy without obvious precipitating factor
- Acute kidney injury or worsening renal function
- Gastrointestinal bleeding
- Shock or hemodynamic instability
- Worsening jaundice or liver function
- Peripheral leukocytosis without clear cause
Laboratory Diagnostic Criteria
Primary Diagnostic Threshold
An ascitic fluid absolute neutrophil count >250 cells/mm³ establishes the diagnosis of SBP. 1, 2 This cutoff was deliberately chosen for its highest sensitivity to avoid missing cases, as delayed treatment increases mortality by 10% for every hour's delay in septic shock. 1, 2
Culture Requirements
Inoculate at least 10 mL of ascitic fluid into blood culture bottles at bedside before administering antibiotics. 1, 2 This technique increases culture sensitivity to >90%. 1, 2 Simultaneously obtain blood cultures before starting antibiotics to increase organism isolation rates. 1, 2
Important caveat: Culture results are NOT required for diagnosis or to initiate treatment—the PMN count alone is sufficient. 1, 2 Culture-negative neutrocytic ascites (PMN >250/mm³ with negative culture) should be treated identically to culture-positive SBP, as both have similar morbidity and mortality. 1, 2
Special Diagnostic Considerations
Bacterascites (Monomicrobial Non-Neutrocytic Ascites)
When cultures are positive but PMN count is <250/mm³: 1, 2
- If symptomatic (fever, abdominal pain): Treat as SBP
- If asymptomatic: Repeat paracentesis, as 38% will progress to frank SBP 2
- Many cases resolve spontaneously through natural defense mechanisms 1
Secondary Bacterial Peritonitis
Suspect secondary (not spontaneous) peritonitis when: 1
- Localized abdominal symptoms or signs
- Multiple organisms on culture
- Very high ascitic neutrophil count
- High ascitic protein concentration
- Inadequate response to appropriate therapy
Perform CT imaging and consider surgical consultation in these cases. 1, 2
Microbiological Patterns
The most common organisms are gram-negative bacteria (~60%), particularly Escherichia coli and Klebsiella pneumoniae. 1 However, there has been a shift toward gram-positive organisms and multidrug-resistant organisms (35% of infections), particularly in nosocomial and healthcare-associated SBP. 1, 3, 4 Spontaneous infections are typically monomicrobial, with fungi representing <5% of cases. 1
Rapid Diagnostic Alternatives
Leukocyte esterase reagent strips can provide rapid bedside diagnosis if available, with sensitivity ranging from 83-100% and specificity from 92.5-100% across studies. 2, 5 Flow cytometry has demonstrated 100% sensitivity and specificity compared to 65.52% sensitivity for manual counting. 6 However, these methods are not yet universally recommended as standard practice, and manual microscopy remains the gold standard. 1
Critical Pitfall to Avoid
Never delay antibiotic therapy while waiting for culture results. 1, 2 The PMN count >250/mm³ is sufficient to initiate empirical antibiotics immediately after obtaining samples for culture. 1, 2 In patients with septic shock, each hour of delay increases mortality by 10%. 1