What are the WBC (white blood cell) count criteria for diagnosing spontaneous bacterial peritonitis (SBP)?

WBC Criteria for Diagnosing Spontaneous Bacterial Peritonitis (SBP)

The diagnosis of SBP is confirmed when the ascitic fluid neutrophil count is >250 cells/mm³ in the absence of an intra-abdominal surgically treatable source of infection. 1

Diagnostic Criteria and Accuracy

Primary Diagnostic Threshold

• Neutrophil count >250 cells/mm³: This is the established diagnostic threshold with the highest sensitivity for SBP diagnosis 1
• This cutoff was specifically chosen to avoid leaving SBP untreated, as delayed treatment significantly increases mortality 1

Alternative Thresholds and Their Accuracy

• Neutrophil count >500 cells/mm³: Has greater specificity but lower sensitivity 1
• Total WBC >1000 cells/μL: Most accurate with positive likelihood ratio of 9.1 (95% CI 5.5 to 15.1) 1
• PMN ≥500 cells/μL: Most accurate with positive likelihood ratio of 10.6 (95% CI 6.1 to 18.3) 1
• WBC >500 cells/μL: Positive likelihood ratio of 5.9 (95% CI 2.3 to 15.5) 1
• PMN >250 cells/μL: Positive likelihood ratio of 6.4 (95% CI 4.6 to 8.8) 1

Diagnostic Algorithm

1. Perform diagnostic paracentesis in all patients with:

   • Cirrhosis and ascites at hospital admission (even without symptoms) 1
   • Gastrointestinal bleeding 1
   • Fever or other signs of systemic inflammation 1
   • Shock 1
   • Worsening liver and/or renal function 1
   • Hepatic encephalopathy 1
   • Gastrointestinal symptoms 1

2. Collect samples properly:

   • Bedside inoculation of at least 10 mL of ascitic fluid into blood culture bottles (increases culture sensitivity to >90%) 1
   • Obtain simultaneous blood cultures 1

3. Interpret results:

   • SBP confirmed: Neutrophil count >250 cells/mm³ regardless of culture results 1
   • Culture-negative neutrocytic ascites: Neutrophil count >250 cells/mm³ with negative culture (treat as SBP) 1
   • Bacterascites: Positive culture but neutrophil count <250 cells/mm³ 1
     • If symptomatic: Treat with antibiotics 1
     • If asymptomatic: Repeat paracentesis when culture results return 1
       • If repeat neutrophil count >250/mm³: Treat for SBP 1
       • If repeat neutrophil count remains <250/mm³: Follow up without treatment 1

Important Clinical Considerations

Culture Results

• Ascitic fluid culture is often negative (up to 60% of cases) despite clinical SBP 1
• Culture results are not necessary for diagnosis but are important to guide antibiotic therapy 1
• Most common pathogens:
  • Gram-negative bacteria (~60%), especially E. coli and Klebsiella 1
  • Gram-positive cocci (streptococcus species and enterococci) 1
  • Recent shift toward gram-positive and multidrug-resistant organisms in nosocomial infections 1

Secondary Bacterial Peritonitis Differentiation

• Suspect secondary bacterial peritonitis (requiring surgical intervention) when:
  • Multiple organisms on ascitic culture 1
  • Very high ascitic neutrophil count 1
  • High ascitic protein concentration (>10 g/L, often >30 g/L) 2
  • Elevated lactate dehydrogenase (>240 U/L) 2
  • Low glucose (<2.7 mmol/L) 2
  • Inadequate response to therapy 1

Common Pitfalls to Avoid

1. Delayed diagnosis: Each hour of delay in diagnostic paracentesis after admission is associated with a 3.3% increase in hospital mortality 1
2. Relying on symptoms alone: Up to one-third of patients with SBP may be asymptomatic 1, and 16% may have completely asymptomatic course 2
3. Waiting for culture results before treatment: Empirical antibiotic therapy must be initiated immediately after diagnosis based on cell count 1
4. Using reagent strips alone: Despite their convenience, reagent strips have low sensitivity and high risk of false negatives, particularly in patients with low neutrophil counts 1
5. Missing secondary bacterial peritonitis: Failure to identify surgically treatable causes of peritonitis can lead to poor outcomes 1

By promptly performing diagnostic paracentesis and applying the >250 neutrophils/mm³ threshold, clinicians can diagnose SBP early and initiate life-saving treatment without delay.