What are the diagnostic criteria and treatment guidelines for peritonitis based on ascites fluid analysis?

Diagnostic Criteria for Peritonitis on Ascites Fluid Analysis

Spontaneous bacterial peritonitis (SBP) is diagnosed when the ascitic fluid polymorphonuclear (PMN) leukocyte count exceeds 250 cells/mm³, regardless of culture results, and empirical antibiotic therapy should be initiated immediately. 1

Primary Diagnostic Threshold

• PMN count >250 cells/mm³ is the gold standard for SBP diagnosis and mandates immediate empirical antibiotic treatment without waiting for culture results 1, 2
• This threshold applies whether ascitic fluid cultures are positive, negative, or pending 1
• Manual microscopy or automated flow cytometry can be used for cell counting and differentiation 1

When to Perform Diagnostic Paracentesis

All cirrhotic patients with ascites require diagnostic paracentesis at hospital admission, even without symptoms of infection 1, 2

Additional indications for urgent paracentesis include: 1, 2

• Fever (temperature >37.8°C or 100°F) or signs of systemic inflammation
• Abdominal pain or tenderness
• Gastrointestinal bleeding or shock
• Hepatic encephalopathy
• Worsening liver or renal function
• Unexplained acidosis or peripheral leukocytosis

Treatment Criteria Based on PMN Count and Clinical Context

PMN >250/mm³

• Immediate empirical antibiotics required regardless of symptoms or culture results 1, 2
• First-line treatment: Cefotaxime 2g IV every 6-8 hours for 5 days 1, 2, 3
• Alternative: Ceftriaxone 1g IV every 24 hours 1
• Add IV albumin 1.5 g/kg at diagnosis, then 1 g/kg on day 3 to reduce hepatorenal syndrome and mortality 1, 2, 3

PMN <250/mm³ WITH Symptoms

• Empirical antibiotics recommended while awaiting culture results if patient has fever, abdominal pain/tenderness, or unexplained complications 1
• Use same antibiotic regimen as for PMN >250/mm³ 1
• This prevents progression to SBP, as many symptomatic patients with PMN <250/mm³ develop full SBP 1

PMN <250/mm³ WITHOUT Symptoms (Monomicrobial Non-Neutrocytic Bacterascites)

• No treatment required as most cases resolve spontaneously 1
• Represents bacterial colonization rather than infection 1

Distinguishing Secondary Bacterial Peritonitis

Secondary peritonitis has 50-80% mortality and requires surgical evaluation, making differentiation from SBP critical 1

Suspect secondary peritonitis when: 1, 4

• PMN count >1,000/mm³
• Multiple organisms on Gram stain or culture
• Ascitic total protein ≥1 g/dL
• Ascitic LDH above normal serum upper limit
• Ascitic glucose ≤50 mg/dL
• PMN count fails to decrease after 48 hours of antibiotics
• Ascitic CEA >5 ng/mL or alkaline phosphatase >240 U/L (suggests intestinal perforation) 1

When secondary peritonitis is suspected, obtain abdominal CT imaging immediately and add anaerobic coverage to antibiotics 1

Essential Ascitic Fluid Tests at Diagnosis

Initial paracentesis for new-onset ascites: 1

• Cell count with differential (PMN count)
• Total protein
• Serum-ascites albumin gradient (SAAG)
• Culture (bedside inoculation into blood culture bottles) 1, 2

When SBP suspected (PMN >250/mm³): 1

• All above tests PLUS:
• Glucose
• LDH
• Gram stain
• Consider CEA and alkaline phosphatase if secondary peritonitis suspected

Monitoring Treatment Response

• Repeat paracentesis at 48 hours to assess treatment efficacy, especially if clinical response inadequate or secondary peritonitis suspected 1, 2, 3
• Treatment success: PMN count decreases by ≥25% from baseline with clinical improvement 1, 2, 3, 4
• Treatment failure: PMN count fails to decrease by 25% or worsening symptoms suggests resistant organisms or secondary peritonitis 1, 2, 3, 4

Critical Pitfalls to Avoid

• Never delay antibiotics waiting for culture results when PMN >250/mm³—mortality increases 10% for every hour of delay in septic patients 2
• Do not rely on reagent strip tests for rapid SBP diagnosis due to low sensitivity and high false-negative rates 1
• Avoid quinolones in patients already on quinolone prophylaxis, in nosocomial SBP, or in areas with high quinolone resistance 1, 2
• Do not omit albumin therapy in patients with baseline bilirubin ≥4 mg/dL or creatinine ≥1 mg/dL, as this significantly reduces mortality from 29% to 10% 1, 3
• Unnecessary laparotomy in cirrhotic patients increases mortality—thoroughly evaluate for secondary peritonitis before surgical consultation 1