What are the differential diagnoses of peritonitis?

Differential Diagnoses of Peritonitis

Peritonitis is classified into three main infectious categories—primary, secondary, and tertiary—plus sterile peritonitis, with the differential diagnosis fundamentally determined by whether gastrointestinal integrity is intact and the clinical context of acquisition. 1

Primary Classification Framework

Infectious Peritonitis

Primary Peritonitis (Spontaneous Bacterial Peritonitis)

• Diffuse bacterial infection WITHOUT loss of gastrointestinal tract integrity 1

• Usually caused by a single organism 1

• Key populations:

  • Cirrhotic patients with ascites (most common)
  • Patients with peritoneal dialysis catheters
  • Nephrotic syndrome patients (especially children) 2
  • Disseminated lupus patients 2
  • Rarely: alcoholic hepatitis, acute viral hepatitis, malignant ascites, pancreatitis 2

• Common organisms: E. coli, Klebsiella pneumoniae, Streptococcus spp., Enterococcus spp. 3, 4

• Diagnostic criteria: PMN count >250/mm³ in ascitic fluid 3, 5

• Low incidence on surgical wards; usually managed without surgical intervention 1

Secondary Peritonitis

• Most common form of peritonitis 1

• Acute peritoneal infection resulting FROM loss of gastrointestinal tract integrity 1, 6

• Causes include:

  • Perforated viscus (duodenal ulcer, diverticular perforation)
  • Gangrenous appendicitis
  • Anastomotic dehiscence (postoperative)
  • Ischemic bowel
  • Traumatic perforations
  • Infected intra-abdominal viscera 6

• Polymicrobial infection: typically 2-3 aerobic species and 1-2 anaerobic species 4

• Common organisms: E. coli, Streptococcus, B. fragilis, Pseudomonas (depending on GI disruption level), Enterococcus, Enterobacter 4

Distinguishing secondary from primary peritonitis in cirrhotic patients: 3

• PMN count >1,000/mm³
• Multiple organisms on Gram stain or culture
• Ascitic total protein ≥1 g/dL
• Ascitic LDH > normal upper limit of serum LDH
• Ascitic glucose ≤50 mg/dL
• PMN count fails to drop after 48 hours of antibiotics
• Elevated ascitic CEA (>5 ng/mL) or alkaline phosphatase (>240 U/L) suggests intestinal perforation
• CT imaging mandatory when suspected 3

Tertiary Peritonitis (Persistent/Ongoing Peritonitis)

• Recurrent infection >48 hours after apparently successful surgical source control of secondary peritonitis 1
• Represents evolution/complication of secondary peritonitis, not a distinct disease entity 1
• Key features:
  • More common in critically ill or immunocompromised patients
  • Associated with multidrug-resistant organisms (MDROs)
  • May include S. epidermidis and Candida spp. 4
  • High morbidity and mortality 1

Healthcare-Associated vs. Community-Acquired

Healthcare-associated IAI (HCAI): 1

• Nursing facility residents
• Recent hospitalization within 90 days
• Aggressive home medical therapies (IV therapy, wound dressing)
• Invasive outpatient therapies within 30 days (hemodialysis, chemotherapy, radiotherapy)
• Higher likelihood of MDROs and Candida infections 1

Post-operative peritonitis: Associated with increased mortality due to underlying comorbidity, atypical presentation with non-specific signs, and MDRO risk 1

Sterile (Non-Infectious) Peritonitis

Depending on underlying pathology, peritonitis can be infectious or sterile 1

Causes include: 7, 8

• Chemical peritonitis
• Eosinophilic peritonitis
• Malignancy-related (peritoneal carcinomatosis) 9
• Myocardial infarction with pericarditis (in dialysis patients) 8
• Endotoxin-contaminated dialysates 8
• Up to 20% of peritoneal dialysis peritonitis cases are culture-negative 7

Special Populations

Peritoneal Dialysis Patients

• PD-related peritonitis 10, 11, 12
• Perforative peritonitis (often confused with PD-related peritonitis; requires contrast-enhanced CT within 5 days if refractory to antibiotics) 10
• Uremic pseudoperitonitis (differentiated by elevated β2-microglobulin vs. CRP elevation in true peritonitis) 13

Tuberculosis Peritonitis

• Often not considered initially 14
• Common diagnostic pitfalls:
  • Negative 5-TU PPD test
  • Normal chest X-ray
  • Low ascitic fluid protein
• Consider when: ascites + fever + abdominal pain, especially with alcoholism, lung lesion, weight loss, or cirrhosis 14

Critical Diagnostic Approach

For cirrhotic patients with ascites: 5

• First episode: SAAG, PMN count, culture, protein concentration
• Recurrent ascites: PMN count, culture (if inpatient)
• SAAG ≥1.1 g/dL = portal hypertension (97% accuracy) 5
• SAAG <1.1 g/dL = peritoneal carcinomatosis, TB peritonitis, other causes 5

Common pitfall: Failure to thoroughly evaluate nonmalignant exudative ascites, especially in alcoholics 14. The washing effect of peritoneal dialysate may relieve peritoneal irritation, making diagnosis of perforative peritonitis difficult in PD patients 10.