What labs should be ordered in peritonitis follow-up?

Laboratory Tests for Peritonitis Follow-Up

In peritonitis follow-up, repeat paracentesis with ascitic fluid analysis should be performed if the clinical setting, symptoms, organism(s), or response to treatment are atypical to distinguish between spontaneous bacterial peritonitis (SBP) and secondary peritonitis. 1

When to Perform Follow-Up Paracentesis

• Routine follow-up paracentesis is not necessary in all patients with infected ascites who show typical presentation and good clinical response to treatment 1
• Follow-up paracentesis should be performed in patients with:
  • Atypical clinical presentation or setting 1
  • Atypical ascitic fluid analysis results 1
  • Unusual or multiple organisms on culture 1
  • Poor clinical response to appropriate antibiotic therapy 1
  • Suspected secondary peritonitis (surgically treatable intra-abdominal source) 1

Laboratory Tests to Order in Follow-Up Paracentesis

Essential Tests:

• Polymorphonuclear leukocyte (PMN) count 1

  • The 48-hour PMN count should decrease from pretreatment value in SBP with appropriate antibiotic therapy
  • Rising PMN count despite treatment suggests nonperforation secondary peritonitis 1

• Gram stain and culture 1

  • To document sterility of culture in responding patients 1
  • To identify persistent or new organisms in non-responders 1

• Total protein 1

  • Values >1 g/dL may suggest secondary peritonitis when combined with other criteria 1

• Lactate dehydrogenase (LDH) 1

  • Values greater than the upper limit of normal for serum may indicate secondary peritonitis 1

• Glucose 1

  • Values <50 mg/dL may suggest secondary peritonitis 1

Additional Tests (if secondary peritonitis is suspected):

• Carcinoembryonic antigen (CEA) 1

  • Values >5 ng/mL suggest gut perforation (92% sensitivity, 88% specificity) 1

• Alkaline phosphatase 1

  • Values >240 U/L suggest gut perforation (92% sensitivity, 88% specificity) 1

Monitoring Algorithm for Peritonitis Follow-Up

1. Initial assessment (24-48 hours after starting antibiotics):

   • Evaluate clinical response (fever, abdominal pain, hemodynamic parameters) 1
   • Monitor serum electrolytes, creatinine, and complete blood count 2

2. Decision for follow-up paracentesis:

   • If typical SBP with good clinical response: follow-up paracentesis is not necessary 1
   • If atypical presentation or poor response: perform follow-up paracentesis with complete fluid analysis 1

3. Interpretation of follow-up paracentesis results:

   • Improving SBP: Decreasing PMN count, negative culture 1
   • Suspected secondary peritonitis: Consider if:
     • PMN count rising despite antibiotics 1
     • Multiple organisms on culture 1
     • At least two of: total protein >1 g/dL, LDH > upper limit of normal, glucose <50 mg/dL 1
     • CEA >5 ng/mL or alkaline phosphatase >240 U/L (for gut perforation) 1

4. Management based on follow-up results:

   • Improving SBP: Continue current antibiotic therapy 1
   • Suspected secondary peritonitis: Add anaerobic coverage to antibiotics and consider surgical evaluation 1

Common Pitfalls and Caveats

• Waiting too long to perform follow-up paracentesis in patients with poor clinical response can delay diagnosis of secondary peritonitis 1
• The classic criteria for secondary peritonitis (protein, LDH, glucose) have 100% sensitivity but only 45% specificity for perforation 1
• The total protein, LDH, and glucose criteria are only 50% sensitive in detecting nonperforation secondary peritonitis; follow-up PMN count is more valuable 1
• Performing follow-up paracentesis too early (before 48 hours of appropriate antibiotic therapy) may lead to misleading results 1
• Automated cell counting in peritoneal fluid may be a reliable approach for early screening of patients with suspected peritonitis 3