What is the diagnostic approach for Spontaneous Bacterial Peritonitis (SBP)?

How to Diagnose Spontaneous Bacterial Peritonitis (SBP)

Perform diagnostic paracentesis immediately in all hospitalized cirrhotic patients with ascites, even without symptoms, and diagnose SBP when ascitic fluid polymorphonuclear (PMN) leukocyte count exceeds 250 cells/mm³—culture positivity is not required for diagnosis. 1

When to Perform Diagnostic Paracentesis

Mandatory indications for immediate paracentesis include: 1

• All cirrhotic patients with ascites at hospital admission, regardless of symptoms (up to one-third of SBP cases are completely asymptomatic) 1
• Gastrointestinal bleeding 1
• Shock or hemodynamic instability 1
• Fever or any signs of systemic inflammation (hypothermia, chills, altered white blood cell count, tachycardia, tachypnea) 1
• Abdominal pain or gastrointestinal symptoms (vomiting, diarrhea, ileus) 1
• Worsening liver function 1
• Acute kidney injury or worsening renal function 1
• New or worsening hepatic encephalopathy 1

Critical pitfall: Delaying paracentesis increases mortality—each hour of delay in antibiotic initiation increases mortality by 10% in cirrhotic patients with septic shock. 2 Do not wait for clinical symptoms to develop, as 16% of SBP cases are entirely asymptomatic. 2

Diagnostic Criteria for SBP

The diagnosis is established by: 1

• Ascitic fluid PMN count >250 cells/mm³ (this is the definitive diagnostic threshold) 1
• PMN count determined by microscopy (flow cytometry-based automated counts are acceptable alternatives) 1
• Culture positivity is NOT required for diagnosis—treat culture-negative neutrocytic ascites (PMN >250/mm³ with negative culture) identically to culture-positive SBP 1, 2

Why 250 cells/mm³ threshold? This cutoff has the highest sensitivity and is deliberately chosen to avoid missing cases of SBP, which carries 20% hospital mortality. 1, 2 The greater clinical risk lies with underdiagnosing rather than overdiagnosing SBP. 2

Proper Specimen Collection Technique

To maximize diagnostic yield: 1

• Inoculate at least 10 mL of ascitic fluid into aerobic AND anaerobic blood culture bottles at the bedside before administering antibiotics (increases culture sensitivity to >90%) 1, 2
• Obtain simultaneous blood cultures before antibiotic initiation (increases organism isolation rates) 1, 2
• Send fluid for cell count with differential, Gram stain, and culture 1

Regarding reagent strips: Leukocyte esterase urine test strips can provide rapid diagnosis if available, but there is insufficient evidence to recommend them in routine practice over standard microscopy. 1 One study showed 100% sensitivity and 98.9% specificity, but guideline bodies remain cautious. 3

Distinguishing SBP from Secondary Bacterial Peritonitis

Suspect secondary bacterial peritonitis (requiring surgical intervention) when: 1, 4

• Multiple organisms on Gram stain or culture (SBP is typically monomicrobial) 1, 4
• Very high ascitic PMN count (often >1,000/mm³) 1, 4
• High ascitic protein concentration 1
• Localized abdominal symptoms or signs 1
• Inadequate response to appropriate antibiotic therapy 1

If secondary peritonitis is suspected: Obtain urgent CT scanning and early surgical consultation. 1, 4

Special Diagnostic Scenarios

Bacterascites (positive culture but PMN <250/mm³): 1

• If patient has signs of systemic inflammation or infection: treat with antibiotics 1
• If asymptomatic: perform repeat paracentesis when culture results return positive 1
• If repeat PMN count >250/mm³ or culture remains positive: treat as SBP 1
• Note: 38% of bacterascites cases progress to frank SBP 2

Spontaneous bacterial pleural empyema (infected hepatic hydrothorax): 1

• Perform diagnostic thoracentesis when pleural effusion is present with suspected infection 1
• Diagnosis: positive pleural fluid culture AND >250 neutrophils/mm³, OR negative culture AND >500 neutrophils/mm³ in absence of pneumonia 1
• Inoculate pleural fluid into blood culture bottles 1
• Associated with SBP in 50% of cases 1

Microbiological Profile

Expected organisms: 1

• Gram-negative bacteria (~60%): most commonly Escherichia coli, followed by Klebsiella pneumoniae 1
• Gram-positive bacteria: Staphylococcus aureus, Enterococcus faecalis, Enterococcus faecium 1
• Multidrug-resistant organisms represent 35% of overall infections, particularly in nosocomial and healthcare-associated SBP 1
• Fungi represent <5% of spontaneous infections 1

Critical action: Start empirical IV antibiotics immediately when PMN count >250/mm³, without waiting for culture results—delay increases mortality. 1, 2 The PMN count alone is sufficient to initiate treatment. 2