What are the criteria for Spontaneous Bacterial Peritonitis (SBP)?

Criteria for Spontaneous Bacterial Peritonitis (SBP)

The diagnosis of SBP is definitively established when the ascitic fluid polymorphonuclear (PMN) leukocyte count is greater than 250 cells/mm³, regardless of whether the ascitic fluid culture is positive or negative. 1

Diagnostic Criteria

Primary Diagnostic Criteria

• Ascitic fluid PMN count >250 cells/mm³ - This is the cornerstone of SBP diagnosis 1
• Positive ascitic fluid culture (typically monomicrobial) - While helpful for guiding antibiotic therapy, this is not required for diagnosis 1

Clinical Presentation

• Abdominal pain and tenderness on palpation (with or without rebound tenderness)
• Ileus
• Up to one-third of patients may be asymptomatic or present with only:
  • Encephalopathy
  • Acute kidney injury (AKI)
  • Worsening liver function 1

Diagnostic Approach

When to Perform Diagnostic Paracentesis

Paracentesis should be performed in patients with cirrhosis and ascites in the following situations:

• At hospital admission (emergent) even without symptoms of infection 1
• When signs of infection are present (fever, abdominal pain)
• With gastrointestinal bleeding
• With shock or signs of systemic inflammation
• With worsening liver or renal function
• With hepatic encephalopathy 1

Proper Technique

• Collect ascitic fluid for cell count and culture before starting antibiotics 1
• Bedside inoculation of at least 10 mL of ascitic fluid into blood culture bottles increases culture sensitivity to >90% 1
• Obtain simultaneous blood cultures to increase the possibility of isolating the causative organism 1

Related Variants and Differential Diagnosis

Bacterascites

• Positive ascitic fluid culture with PMN count <250 cells/mm³ 1, 2
• Management:
  • If patient has signs of infection: treat with antibiotics 1
  • If asymptomatic: repeat paracentesis when culture results return positive
  • If repeat PMN count >250/mm³: treat for SBP
  • If repeat PMN count remains <250/mm³: follow up (may resolve spontaneously in 62% of cases) 1, 2

Spontaneous Bacterial Pleural Empyema (SBPE)

• Infection of pre-existing hepatic hydrothorax
• Diagnostic criteria:
  • Positive pleural fluid culture and PMN count >250 cells/mm³, OR
  • Negative pleural fluid culture and PMN count >500 cells/mm³
  • Absence of pneumonia 1

Secondary Bacterial Peritonitis

Must be distinguished from SBP as it requires surgical intervention. Suspect when:

• Multiple organisms on ascitic culture
• Very high ascitic neutrophil count
• High ascitic protein concentration (>1 g/dL)
• At least two of the following:
  • Ascitic total protein >1 g/dL
  • Lactate dehydrogenase (LDH) > upper limit of normal for serum
  • Glucose <50 mg/dL 1
• Inadequate response to antibiotic therapy 1

Common Pitfalls and Caveats

1. Delayed diagnosis: SBP can be asymptomatic in up to one-third of patients. Always perform diagnostic paracentesis in hospitalized cirrhotic patients with ascites, even without symptoms 1

2. Culture-negative SBP: Many cases of SBP have negative cultures despite PMN count >250/mm³. These should still be treated as SBP 1

3. Delayed treatment: Mortality increases by 10% for every hour's delay in initiating antibiotics in patients with cirrhosis and septic shock 1

4. Misdiagnosis of secondary peritonitis: Failure to identify secondary peritonitis requiring surgical intervention can lead to poor outcomes. Consider secondary peritonitis when multiple organisms are present or when there's inadequate response to therapy 1

5. Failure to obtain cultures: Always obtain ascitic fluid cultures before starting antibiotics to guide therapy, especially given the increasing prevalence of multidrug-resistant organisms 1

6. Relying only on automated cell counters: While automated counts can be used, microscopic examination remains the gold standard for PMN determination 1

By following these criteria and diagnostic approaches, SBP can be promptly diagnosed and treated, significantly improving patient outcomes and reducing mortality.