What are the diagnostic criteria for Spontaneous Bacterial Peritonitis (SBP) in patients with cirrhosis and ascites?

Diagnostic Criteria for Spontaneous Bacterial Peritonitis (SBP)

SBP is diagnosed when ascitic fluid absolute polymorphonuclear (PMN) leukocyte count is >250 cells/mm³, regardless of culture results, in the absence of an intra-abdominal surgically treatable source of infection. 1, 2

Core Diagnostic Requirements

The diagnosis requires two essential components:

• Ascitic fluid PMN count >250 cells/mm³ - This threshold was deliberately chosen for its high sensitivity to avoid missing cases, as each hour of treatment delay increases mortality by 3.3% 2, 3
• Exclusion of secondary peritonitis - No evidence of intra-abdominal surgically treatable infection source (e.g., perforated viscus, abscess) 1

The diagnosis is valid even when cultures are negative, as culture-negative neutrocytic ascites has identical clinical outcomes and mortality to culture-positive SBP and should be treated identically 2, 3

When to Perform Diagnostic Paracentesis

Paracentesis is mandatory in the following situations:

• All cirrhotic patients with ascites at hospital admission, even without symptoms, as up to one-third of SBP cases are completely asymptomatic 1, 2, 3
• Any clinical deterioration: fever, abdominal pain, altered mental status/hepatic encephalopathy, acute kidney injury, worsening liver function, gastrointestinal bleeding, shock, or peripheral leukocytosis without obvious cause 1, 3

Proper Specimen Collection Technique

To maximize diagnostic yield:

• Inoculate at least 10 mL of ascitic fluid into blood culture bottles at the bedside before starting antibiotics - this increases culture sensitivity to >90% 2, 3
• Obtain simultaneous blood cultures before antibiotic initiation to increase organism isolation rates 3
• Send ascitic fluid for cell count with differential, total protein, albumin, and Gram stain 1

Critical Pitfall: Do Not Wait for Culture Results

Empiric antibiotic therapy must be initiated immediately when PMN count >250 cells/mm³, without waiting for culture results 2, 3. In patients with septic shock from suspected SBP, mortality increases by 10% for every hour's delay in initiating antibiotics 2. The PMN count alone is sufficient to diagnose and treat 3.

SBP Variants Requiring Clinical Judgment

Culture-Negative Neutrocytic Ascites (CNNA)

• PMN >250 cells/mm³ with negative culture 1
• Treat identically to culture-positive SBP 2, 3
• Occurs in 86% of cases if even a single antibiotic dose was given before paracentesis 1

Monomicrobial Bacterascites

• Positive culture but PMN <250 cells/mm³ 1
• If symptomatic or signs of systemic inflammation: treat with antibiotics 2
• If asymptomatic: repeat paracentesis when culture results return, as 38% will progress to frank SBP 1, 3
• Most cases resolve spontaneously through natural defense mechanisms 1

Alternative Thresholds (For Context Only)

While >250 cells/mm³ is the standard diagnostic threshold 1, 2, some data show:

• PMN ≥500 cells/mm³ has the highest positive likelihood ratio (10.6) for confirming SBP 3
• However, the lower threshold of >250 cells/mm³ is deliberately maintained because the greater clinical risk lies with underdiagnosing SBP rather than overdiagnosing it 3

Distinguishing Secondary Bacterial Peritonitis

Secondary bacterial peritonitis should be suspected when:

• Ascitic fluid total protein >10 g/L (especially >30 g/L) 4
• Ascitic fluid LDH >240 U/L 4
• Ascitic fluid glucose <2.7 mmol/L 4
• Polymicrobial flora or isolation of anaerobes, enterococci, or fungi 4
• Extremely high PMN counts with refractoriness to appropriate antibiotic therapy 4
• CT imaging should be obtained if secondary peritonitis is suspected 4