Diagnosing Spontaneous Bacterial Peritonitis (SBP)
The diagnosis of SBP is established by performing a diagnostic paracentesis and finding an ascitic fluid absolute neutrophil count >250 cells/mm³, regardless of culture results. 1
When to Perform Diagnostic Paracentesis
Perform diagnostic paracentesis immediately in ALL cirrhotic patients with ascites upon hospital admission, even without any symptoms of infection. 1 This is a critical point—do not wait for symptoms to develop, as up to one-third of patients with SBP are completely asymptomatic. 1
Additionally, perform urgent diagnostic paracentesis in patients who develop: 1
- Fever or hypothermia
- Abdominal pain or tenderness
- Hepatic encephalopathy
- Acute kidney injury or worsening renal function
- Peripheral leukocytosis without obvious cause
- Gastrointestinal bleeding
- Shock or hemodynamic instability
- Jaundice or worsening liver function
Diagnostic Criteria
The diagnosis requires: 1
- Ascitic fluid absolute neutrophil (PMN) count >250 cells/mm³
- Absence of an intra-abdominal surgically treatable source of infection
Culture results are NOT required for diagnosis—treat based on the cell count alone. 1 Approximately 40-50% of SBP cases will have negative cultures even when performed correctly. 1
Proper Specimen Collection Technique
Inoculate at least 10 mL of ascitic fluid into blood culture bottles at the bedside BEFORE starting antibiotics. 1 This bedside inoculation technique increases culture sensitivity to >90%. 1
Simultaneously obtain blood cultures to increase the likelihood of isolating a causative organism. 1
Clinical Presentation Patterns
Be aware that SBP presents variably: 1
- Classic symptoms: abdominal pain, fever, tenderness (with or without rebound), ileus
- Subtle presentations: isolated encephalopathy or acute kidney injury
- Completely asymptomatic in up to one-third of cases
Critical Timing Consideration
In patients with septic shock from suspected SBP, mortality increases by 10% for every hour's delay in initiating antibiotics. 1 Therefore, start empirical antibiotics immediately after obtaining cultures if clinical suspicion is high, particularly with hemodynamic instability. 1
Special Variants to Recognize
Culture-negative neutrocytic ascites (PMN ≥250/mm³ with negative culture): Treat identically to culture-positive SBP, as clinical outcomes and mortality are similar. 1
Monomicrobial bacterascites (positive culture but PMN <250/mm³): 1
- If symptomatic or signs of systemic inflammation: treat with antibiotics 1
- If asymptomatic: repeat paracentesis when culture results return
- Treat if repeat PMN >250/mm³; otherwise observe closely 1
Common Pitfalls to Avoid
Do not delay paracentesis due to coagulopathy concerns—routine correction of INR or platelet transfusion is not recommended unless platelets are <40,000-50,000/μL. 2, 3 The perceived bleeding risk should not prevent this life-saving diagnostic procedure. 2
Do not wait for culture results to initiate treatment—the diagnosis is based on cell count, and cultures are primarily useful for guiding antibiotic therapy adjustments. 1