Diagnostic Approach for Spontaneous Bacterial Peritonitis
All cirrhotic patients with ascites who are emergently admitted to the hospital must undergo immediate diagnostic paracentesis to rule out SBP, even without symptoms of infection 1.
When to Perform Diagnostic Paracentesis
Perform diagnostic paracentesis immediately in these situations:
- Emergency hospital admission with ascites (regardless of symptoms) 1
- Development of any signs/symptoms suggesting infection (fever, abdominal pain, altered mental status, diarrhea) 1
- Laboratory abnormalities suggesting infection 1
- Tense ascites with acute kidney injury 1
- Unexplained clinical deterioration 1
Diagnostic Criteria
The diagnosis of SBP is established when ascitic fluid polymorphonuclear (PMN) leukocyte count exceeds 250 cells/mm³ 1, 2. This threshold is diagnostic regardless of culture results.
Key Diagnostic Steps:
Obtain ascitic fluid sample via paracentesis before any antibiotics are given
Send for immediate cell count with differential - PMN count >250/mm³ confirms SBP 1
Inoculate blood culture bottles at bedside - Use at least 10 mL of ascitic fluid into both aerobic and anaerobic blood culture bottles immediately at bedside (increases culture sensitivity to >90%) 1
Obtain simultaneous blood cultures - This increases the likelihood of isolating the causative organism 1
Critical Timing Issues
Start empirical IV antibiotics immediately when PMN count >250/mm³, before culture results return 1. Do not wait for culture confirmation. The culture serves to guide antibiotic narrowing later, not to delay initial treatment.
Common Pitfalls to Avoid
Never delay paracentesis in admitted cirrhotic patients with ascites, even if asymptomatic - SBP can be clinically silent 1
Do not inoculate culture bottles in the lab - Bedside inoculation is essential for adequate sensitivity 1. A novel centrifugation-washing-lysis method shows promise for improving culture yield (30.6% vs 13.8% positivity) and reducing time to detection by 56.5%, though this requires specialized processing 3
Culture-negative SBP is common (up to 56% in some series) but does not change management - treat based on PMN count alone 1, 4
Do not confuse with secondary peritonitis - If PMN >250/mm³ but patient has focal intra-abdominal pathology (diverticulitis, cholecystitis), treat the underlying condition with surgical consultation, not as SBP 1
Expected Microbiology
Spontaneous infections are monobacterial in nature 1:
- Gram-negative bacteria (~60%): Primarily E. coli (most common), followed by Klebsiella pneumoniae 1, 4
- Gram-positive organisms: Increasingly common, especially S. aureus, Enterococcus species 1
- Multidrug-resistant organisms (MDROs): Now represent 35% of infections, particularly in nosocomial and healthcare-associated SBP 1, 4
- Fungi represent <5% of infections 1
Special Considerations for Pleural Effusion
If diagnostic workup is negative for infection but the patient has a pleural effusion, perform diagnostic thoracentesis 1. Spontaneous bacterial empyema (SBE) is diagnosed with pleural fluid PMN >250/mm³ and requires antibiotics but not chest tube drainage 1.
Antibiotic Selection Based on Risk Factors
The choice of empirical antibiotic depends on MDRO risk:
Community-acquired SBP (low MDRO risk):
- Third-generation cephalosporin (cefotaxime 2g IV every 12 hours or ceftriaxone) 1
Nosocomial or healthcare-associated SBP (high MDRO risk):
- Consider broader coverage with carbapenems, especially in:
- Recent hospitalization
- Critically ill/ICU patients
- Septic shock (inappropriate initial antibiotics increase mortality 10-fold) 1
Narrow antibiotics as soon as culture and sensitivity results are available to practice antibiotic stewardship 1.