What is the most suggestive indicator of spontaneous bacterial peritonitis (SBP)?

Diagnosis of Spontaneous Bacterial Peritonitis (SBP)

An ascitic fluid neutrophil count >250/mm³ is the most suggestive indicator of spontaneous bacterial peritonitis (SBP), regardless of culture results. 1

Diagnostic Criteria for SBP

The diagnosis of SBP is firmly established based on the following criteria:

• Neutrophil count >250/mm³ in ascitic fluid - This is the primary diagnostic criterion with highest sensitivity
• No evidence of surgically treatable intra-abdominal source of infection
• Culture results are supportive but not required for diagnosis

Key Ascitic Fluid Parameters

Parameter	SBP Finding	Secondary Peritonitis Finding
Neutrophil count	>250/mm³	Often >1000/mm³
Culture	Typically monobacterial (if positive)	Often polymicrobial
Protein	Usually <1 g/dL	Often >1 g/dL
Glucose	Usually normal	Often <50 mg/dL

Variants of Ascitic Fluid Infection

1. Classic SBP:

   • Neutrophil count >250/mm³
   • Positive culture (typically monobacterial)
   • Most commonly caused by gram-negative enteric bacteria (E. coli, Klebsiella)

2. Culture-negative neutrocytic ascites:

   • Neutrophil count >250/mm³
   • Negative culture
   • Represents approximately 40% of SBP cases 1
   • Clinically managed the same as culture-positive SBP

3. Bacterascites (Monomicrobial non-neutrocytic bacterascites):

   • Neutrophil count <250/mm³
   • Positive culture (single organism)
   • May represent early SBP or transient colonization 2, 3
   • Requires follow-up paracentesis and treatment if symptoms present

Clinical Approach to Diagnosis

1. Mandatory diagnostic paracentesis in:

   • All cirrhotic patients with ascites at hospital admission
   • Patients with signs of infection (fever, abdominal pain)
   • Worsening liver or renal function
   • Hepatic encephalopathy
   • GI bleeding
   • Shock 1

2. Proper sampling technique:

   • Bedside inoculation into blood culture bottles increases culture yield to >90% (vs ~50% with conventional methods) 1
   • Obtain at least 10 mL of ascitic fluid for culture
   • Collect blood cultures simultaneously to increase pathogen identification 1

3. Laboratory assessment:

   • Cell count with differential (neutrophil count)
   • Culture (aerobic and anaerobic)
   • Protein, glucose, LDH levels (to help differentiate from secondary peritonitis)

Important Clinical Pearls

• Delay in diagnostic paracentesis is associated with increased mortality (3.3% increase in mortality per hour of delay) 1, 2
• Clear ascitic fluid appearance does not reliably exclude SBP (sensitivity of abnormal appearance is 98.1%) 4
• Secondary bacterial peritonitis should be suspected with:
  • Multiple organisms on culture
  • Very high neutrophil count (>1000/mm³)
  • High ascitic protein concentration
  • Low glucose (<50 mg/dL)
  • Inadequate response to antibiotics 1

Management Implications

• Empiric antibiotic therapy must be initiated immediately after diagnosis of SBP (neutrophil count >250/mm³)
• Third-generation cephalosporins remain first-line treatment in settings where multi-drug resistant organisms are not prevalent 1
• Albumin administration should be considered, particularly in patients with renal dysfunction or high risk of hepatorenal syndrome

In summary, while other parameters like culture results, protein levels, and glucose concentrations provide valuable information, the ascitic fluid neutrophil count >250/mm³ remains the single most important and suggestive indicator for diagnosing SBP and should prompt immediate antibiotic treatment.