How to diagnose Spontaneous Bacterial Peritonitis (SBP) in ascitic fluid?

Diagnosis of Spontaneous Bacterial Peritonitis (SBP) in Ascitic Fluid

The diagnosis of SBP is definitively established when the ascitic fluid neutrophil count exceeds 250 cells/mm³, as determined by microscopy or automated cell counting. 1

Diagnostic Approach

When to Perform Diagnostic Paracentesis

• Perform diagnostic paracentesis in:
  • All cirrhotic patients with ascites at hospital admission, even without symptoms 1, 2
  • Patients with gastrointestinal bleeding 1
  • Patients with shock or signs of systemic inflammation 1
  • Patients with gastrointestinal symptoms 1
  • Patients with worsening liver or renal function 1
  • Patients with hepatic encephalopathy 1

This aggressive diagnostic approach is necessary because SBP is often asymptomatic (up to one-third of cases) and delayed diagnosis increases mortality 1, 3.

Sample Collection and Processing

1. Collect ascitic fluid via paracentesis (preferred site: left lower quadrant, 3 cm cephalad and medial to anterior superior iliac spine) 1
2. Send samples for:
   • Cell count with differential (essential for diagnosis) 1, 2
   • Culture (bedside inoculation into blood culture bottles) 1, 2
   • Total protein (to identify SBP risk and differentiate from secondary peritonitis) 1, 2
   • Optional: LDH and glucose (to help differentiate from secondary peritonitis) 2
3. Simultaneously obtain blood cultures 1

Diagnostic Criteria

• Primary diagnostic criterion: Ascitic fluid neutrophil count >250 cells/mm³ 1, 2
• Culture results are not necessary for diagnosis but guide antibiotic therapy 1
• Bedside inoculation of ascitic fluid into blood culture bottles increases culture sensitivity to >80-90% 1

Special Considerations

Bacterascites

• Defined as positive ascitic fluid culture with neutrophil count <250 cells/mm³ 1
• Management:
  • If patient has signs of systemic inflammation: treat with antibiotics 1
  • If asymptomatic: repeat paracentesis when culture results return positive 1
  • If repeat neutrophil count >250 cells/mm³: treat for SBP 1
  • If repeat neutrophil count remains <250 cells/mm³: follow up without antibiotics 1

Secondary Bacterial Peritonitis

• Suspect when:
  • Localized abdominal symptoms or signs are present 1, 2
  • Multiple organisms are seen on ascitic culture 1, 2
  • Very high ascitic neutrophil count 1, 2
  • High ascitic protein concentration 1, 2
  • Inadequate response to therapy 1, 2
• Diagnostic approach:
  • Perform appropriate radiological investigation (CT scan) 1
  • Additional tests: ascitic fluid total protein >1 g/dL, LDH higher than upper limit of normal for serum, glucose <50 mg/dL 2

Spontaneous Bacterial Pleural Empyema

• May complicate hepatic hydrothorax 1
• Diagnostic thoracocentesis should be performed in patients with pleural effusion and suspected infection 1
• Diagnosis based on:
  • Positive pleural fluid culture and neutrophil count >250/mm³, or
  • Negative pleural fluid culture and neutrophil count >500/mm³ in absence of pneumonia 1

Diagnostic Pitfalls to Avoid

1. Relying on gross appearance of ascitic fluid: Poor diagnostic accuracy with sensitivity of only 46.88% for detecting SBP 4

2. Delaying paracentesis: Mortality increases by 10% for every hour's delay in initiating antibiotics in patients with septic shock 1

3. Not sending cultures properly: Bedside inoculation of at least 10 mL of ascitic fluid into blood culture bottles increases sensitivity to >90% 1

4. Missing asymptomatic SBP: Up to one-third of patients with SBP may be entirely asymptomatic 1, 3

5. Not considering SBP variants: Bacterascites and culture-negative neutrocytic ascites require specific management approaches 5

By following this diagnostic algorithm, clinicians can promptly identify and treat SBP, significantly reducing morbidity and mortality in patients with cirrhosis and ascites.