What are the diagnostic criteria and treatment for Spontaneous Bacterial Peritonitis (SBP) in ascitic fluid?

Diagnosis of Spontaneous Bacterial Peritonitis (SBP)

SBP is diagnosed when ascitic fluid polymorphonuclear (PMN) leukocyte count exceeds 250 cells/mm³, regardless of culture results, and empiric antibiotics must be started immediately without waiting for culture confirmation. 1, 2

Diagnostic Criteria

When to Perform Diagnostic Paracentesis

• Perform diagnostic paracentesis in ALL cirrhotic patients with ascites at hospital admission, even without any symptoms or signs of infection. 1, 2

• Diagnostic paracentesis is mandatory when patients develop fever, abdominal pain or tenderness, gastrointestinal symptoms, hepatic encephalopathy, worsening liver or renal function, gastrointestinal bleeding, shock, or any signs of systemic inflammation. 1

• In patients with tense ascites and acute kidney injury, perform paracentesis to exclude SBP as the cause. 1

Laboratory Diagnosis

The diagnostic threshold is ascitic fluid PMN count >250 cells/mm³—this single criterion is sufficient to diagnose SBP and initiate treatment. 1, 2

• This threshold was deliberately chosen for maximum sensitivity to avoid missing cases, as untreated SBP carries high mortality with a 10% increase in death for every hour of delayed antibiotic therapy in septic shock. 1, 2

• PMN count can be determined by manual microscopy or automated blood cell counters using flow cytometry—both are acceptable and have excellent agreement. 1, 3

• Automated cell counters have 94% sensitivity and 100% specificity for diagnosing SBP when using the 250 cells/mm³ cutoff. 3

Culture Requirements

Inoculate at least 10 mL of ascitic fluid into aerobic and anaerobic blood culture bottles at the bedside BEFORE administering any antibiotics. 1, 2

• Bedside inoculation increases culture sensitivity to >90%. 1, 2

• Simultaneously obtain blood cultures before starting antibiotics to increase organism isolation rates. 1, 2

• Critical pitfall: Even a single dose of antibiotics causes culture negativity in 86% of cases, leaving only resistant organisms detectable. 1

Culture-Negative Neutrocytic Ascites

• If PMN count >250 cells/mm³ but culture is negative, this is still SBP and requires identical treatment. 2

• Culture-negative neutrocytic ascites has similar morbidity and mortality to culture-positive SBP. 2

Treatment Algorithm

Immediate Empiric Antibiotic Therapy

Start IV antibiotics immediately once PMN count >250 cells/mm³ is confirmed—do not wait for culture results. 1, 2

First-line treatment: Cefotaxime 2g IV every 8-12 hours for 5 days. 1, 2

• Cefotaxime achieves 77-98% resolution rates and is the most extensively studied regimen. 2

• A 5-day course is as effective as 10 days of treatment. 1, 2

• Alternative for uncomplicated SBP: Oral ofloxacin 400mg twice daily can be used in patients without vomiting, shock, hepatic encephalopathy, or serum creatinine >3 mg/dL. 1, 2

Albumin Administration

Administer IV albumin 1.5 g/kg body weight within 6 hours of diagnosis, followed by 1.0 g/kg on day 3. 2

• This regimen reduces mortality from 29% to 10% and decreases hepatorenal syndrome from 33% to 10%. 1, 2

• Albumin therapy is essential and significantly impacts mortality and renal outcomes. 2

Monitoring Treatment Response

Perform repeat paracentesis at 48 hours to assess treatment efficacy. 1, 2

• Treatment success is defined as a decrease in ascitic PMN count to <25% of the pre-treatment value with clinical improvement. 2

• If PMN count fails to decrease by at least 25%, suspect treatment failure and consider resistant bacteria or secondary bacterial peritonitis. 2

Antibiotic Selection Based on Context

• For community-acquired SBP: Cefotaxime or ceftriaxone as first-line. 1, 2

• For nosocomial or healthcare-associated SBP: Consider broader-spectrum coverage due to increased prevalence of gram-positive organisms and multidrug-resistant organisms (35% of infections). 1, 4

• For patients on quinolone prophylaxis: Use cefotaxime or amoxicillin-clavulanic acid—never use quinolones in patients already taking them for prophylaxis. 2

• In areas with high quinolone resistance: Avoid quinolones and use third-generation cephalosporins. 2

Common Pitfalls to Avoid

• Never make a "clinical diagnosis" of SBP without paracentesis—ascitic fluid analysis is mandatory before confident diagnosis. 1

• Never delay paracentesis or antibiotic therapy—up to one-third of SBP patients are completely asymptomatic or present only with encephalopathy or acute kidney injury. 1, 2

• Do not wait for culture results to start treatment—the PMN count alone is sufficient to initiate empirical antibiotics. 2

• Bacterascites (positive culture but PMN <250 cells/mm³) requires clinical judgment: If symptomatic, treat as SBP; if asymptomatic, repeat paracentesis as 38% will progress to frank SBP. 2

Rapid Diagnostic Adjuncts

• Reagent strips (Multistix or Combur test) can provide rapid bedside diagnosis with 89% sensitivity and 100% specificity, allowing immediate commencement of empirical antibiotics. 5

• Automated cell counts provide results faster than manual microscopy and are equally reliable. 1, 3

Microbiology Patterns

• Historically, gram-negative bacteria (especially E. coli and Klebsiella pneumoniae) accounted for ~60% of infections. 1, 6

• Recent shift toward gram-positive organisms (Staphylococcus aureus, Enterococcus species) and multidrug-resistant organisms, particularly in nosocomial settings. 1, 4

• Spontaneous infections are typically monobacterial; polymicrobial infection suggests secondary bacterial peritonitis requiring surgical evaluation. 1