What are the diagnostic criteria for spontaneous bacterial peritonitis?

Diagnostic Criteria for Spontaneous Bacterial Peritonitis

The diagnosis of spontaneous bacterial peritonitis (SBP) is established when ascitic fluid shows an absolute neutrophil (polymorphonuclear leukocyte) count greater than 250 cells/mm³, regardless of whether the culture is positive or negative. 1, 2

Core Diagnostic Requirements

Perform diagnostic paracentesis immediately in all hospitalized cirrhotic patients with ascites, even without symptoms of infection, as up to one-third of patients may be completely asymptomatic. 1, 2 The procedure should also be done urgently in patients presenting with:

• Fever or signs of systemic inflammation 1
• Abdominal pain or gastrointestinal symptoms 1
• Hepatic encephalopathy 1
• Worsening liver or renal function 1
• Gastrointestinal bleeding or shock 1

The Single Definitive Criterion

An ascitic fluid absolute neutrophil count >250/mm³ confirms the diagnosis and mandates immediate empirical antibiotic treatment. 1, 2 This threshold was deliberately chosen for its high sensitivity to avoid missing cases, as delayed treatment increases mortality by 10% for every hour of delay in septic patients. 2, 3

A higher threshold of 500 neutrophils/mm³ has greater specificity but lower sensitivity—the clinical priority is avoiding underdiagnosis given SBP's high mortality risk. 1, 2

Culture Results Are Not Required for Diagnosis

Ascitic fluid culture is frequently negative (in 20-50% of cases) even when performed correctly, and you should never delay treatment waiting for culture results. 1 However, culture remains essential for guiding antibiotic therapy:

• Inoculate at least 10 mL of ascitic fluid into aerobic and anaerobic blood culture bottles at the bedside before starting antibiotics—this increases culture sensitivity to >90%. 1, 2
• Obtain simultaneous blood cultures before antibiotic initiation to increase organism isolation rates. 1, 2

Variants of Ascitic Fluid Infection

Culture-negative neutrocytic ascites (neutrophils ≥250/mm³ with negative culture) represents 40% of SBP cases and must be treated identically to culture-positive SBP, as clinical outcomes are indistinguishable. 1, 2, 4

Monomicrobial bacterascites (positive culture but neutrophils <250/mm³) requires clinical judgment: 1, 2, 4

• If the patient is symptomatic (fever, abdominal pain, signs of infection), treat immediately as SBP. 1, 2
• If completely asymptomatic, repeat paracentesis when culture results return positive—38% will progress to frank SBP, sometimes within hours. 1, 2, 5
• In asymptomatic cases with persistently low neutrophil counts on repeat tap, many resolve spontaneously without treatment. 1, 5

Critical Pitfall: Excluding Secondary Bacterial Peritonitis

You must differentiate SBP from secondary bacterial peritonitis, which requires surgical intervention rather than antibiotics alone. 1, 3 Suspect secondary peritonitis when:

• Multiple organisms appear on Gram stain or culture (SBP is typically monomicrobial). 1, 3
• Ascitic neutrophil count is very high (often >1,000/mm³). 1, 3
• Localized abdominal symptoms or signs are present. 1
• Very high ascitic protein concentration is found. 1
• Patient fails to respond adequately to appropriate antibiotic therapy. 1

Order CT imaging and obtain surgical consultation immediately if secondary peritonitis is suspected. 1

Additional Diagnostic Considerations

Spontaneous bacterial empyema (infection of hepatic hydrothorax) should be considered when pleural effusion is present—diagnose with pleural fluid neutrophils >250/mm³ with positive culture, or >500/mm³ with negative culture, in the absence of pneumonia. 1, 4

The typical microbiology shows approximately 60% gram-negative bacteria (most commonly E. coli and Klebsiella pneumoniae), with increasing rates of gram-positive organisms and multidrug-resistant bacteria, particularly in nosocomial infections. 1, 6, 7