How is Spontaneous Bacterial Peritonitis (SBP) diagnosed based on peritoneal fluid cell count?

Diagnosis of Spontaneous Bacterial Peritonitis Based on Peritoneal Fluid Cell Count

Spontaneous bacterial peritonitis (SBP) is diagnosed when the ascitic fluid neutrophil count is ≥250 cells/mm³, regardless of whether the bacterial culture is positive or negative. 1

Collection and Processing of Peritoneal Fluid

• Diagnostic paracentesis is mandatory for all cirrhotic patients with ascites at hospital admission to rule out SBP 1
• Paracentesis should also be performed in patients with:
  • Gastrointestinal bleeding
  • Fever or other signs of systemic inflammation
  • Abdominal pain or gastrointestinal symptoms
  • Worsening liver or renal function
  • Hepatic encephalopathy
  • Shock 1

Calculating and Interpreting Neutrophil Count

1. Collection method: Obtain ascitic fluid via paracentesis (left lower quadrant is preferred site - 3 cm cephalad and 3 cm medial to anterior superior iliac spine) 1

2. Processing: Centrifuge the fluid sample and prepare a smear with Giemsa stain for microscopic examination 1, 2

3. Calculation of absolute neutrophil count:

   • Formula: Total WBC count × (% neutrophils ÷ 100) 2
   • Example: If total WBC is 1000 cells/mm³ with 70% neutrophils, absolute neutrophil count = 700 cells/mm³

4. Diagnostic thresholds:

   • ≥250 neutrophils/mm³: Diagnostic of SBP (highest sensitivity) 1
   • ≥500 neutrophils/mm³: Higher specificity but may miss some cases 1

Clinical Scenarios and Interpretation

1. Culture-positive neutrocytic ascites:

   • Neutrophil count ≥250 cells/mm³ with positive bacterial culture
   • Definitive SBP diagnosis 1

2. Culture-negative neutrocytic ascites:

   • Neutrophil count ≥250 cells/mm³ with negative bacterial culture
   • Should be treated as SBP 1

3. Bacterascites:

   • Neutrophil count <250 cells/mm³ with positive bacterial culture
   • Management:
     • If patient has signs of infection/inflammation: Treat as SBP
     • If asymptomatic: Repeat paracentesis when culture results return
     • If repeat neutrophil count ≥250 cells/mm³: Treat as SBP
     • If repeat neutrophil count remains <250 cells/mm³: Follow up 1

Potential Pitfalls and Considerations

• Automated cell counters may be used as they show good correlation with manual counts 1
• Reagent strips (dipsticks) are NOT recommended for rapid diagnosis due to low sensitivity and high risk of false negatives 1, 2
• Clinical impression and ascitic fluid appearance are unreliable for ruling out SBP (sensitivity of clinical impression only 42%) 3
• Secondary bacterial peritonitis should be suspected with:
  • Multiple organisms on culture
  • Very high neutrophil count
  • High ascitic protein concentration
  • Inadequate response to therapy 1

When to Start Treatment

• Empiric antibiotic therapy must be initiated immediately after SBP diagnosis (neutrophil count ≥250 cells/mm³) 1
• Do not wait for culture results before starting antibiotics 1
• Third-generation cephalosporins (cefotaxime) are first-line therapy 1

SBP is a serious complication with high mortality, especially when associated with renal impairment (mortality rates of 36-100% depending on severity of renal dysfunction) 4. Early diagnosis through proper ascitic fluid analysis and prompt treatment are essential for improving outcomes.