What are the key differences in ascitic fluid analysis between tubercular ascites and secondary bacterial peritonitis?

Key Differences in Ascitic Fluid Analysis Between Tubercular Ascites and Secondary Bacterial Peritonitis

Tubercular ascites and secondary bacterial peritonitis have distinct ascitic fluid profiles that can be differentiated through specific laboratory parameters in the ascitic fluid analysis.

Cell Count and Differential

• Tubercular ascites:

  • Typically shows lymphocytic predominance
  • PMN count usually <250/mm³ (though can be higher in early disease)
  • Mononuclear cell predominance in established cases 1

• Secondary bacterial peritonitis:

  • Marked neutrophilic predominance
  • PMN count often >1,000/mm³
  • Multiple organisms on Gram stain and culture 1

Biochemical Parameters

Adenosine Deaminase (ADA)

• Tubercular ascites:

  • Elevated ADA levels (>32-40 U/L)
  • Sensitivity of 91.7-100% and specificity of 92-96.6% 1
  • Lower cutoff (>27 U/L) may be needed in cirrhotic patients 1

• Secondary bacterial peritonitis:

  • Normal or mildly elevated ADA levels
  • Not a characteristic feature 2

Glucose

• Tubercular ascites:

  • Usually >50 mg/dL
  • May be decreased but not as dramatically as in secondary peritonitis 3

• Secondary bacterial peritonitis:

  • Markedly decreased (<50 mg/dL)
  • Low glucose is a key diagnostic feature 1

Lactate Dehydrogenase (LDH)

• Tubercular ascites:

  • LDH levels typically <110 U/L 2
  • Lower than in secondary peritonitis

• Secondary bacterial peritonitis:

  • LDH level higher than the upper limit of normal for serum
  • Elevated LDH is one of the diagnostic criteria 1

Total Protein

• Tubercular ascites:

  • Usually >1 g/dL (exudative)
  • Higher protein content than typical cirrhotic ascites 1

• Secondary bacterial peritonitis:

  • Total protein ≥1 g/dL
  • Part of the diagnostic criteria for secondary peritonitis 1

pH

• Tubercular ascites:

  • Decreased pH compared to cirrhotic sterile ascites
  • Arterial blood-ascitic fluid pH gradient >0.10 3

• Secondary bacterial peritonitis:

  • Also shows decreased pH
  • Not specifically useful for differentiation from tubercular ascites 3

Additional Markers

• Secondary bacterial peritonitis specific markers:

  • Elevated CEA (>5 ng/mL)
  • Elevated alkaline phosphatase (>240 U/L) 1
  • These are particularly useful for detecting gut perforation

• Tubercular ascites:

  • Positive AFB smear (sensitivity 0-86%)
  • Positive culture for mycobacteria (sensitivity 20-83%)
  • PCR for mycobacteria has higher sensitivity 1

Treatment Response

• Tubercular ascites:

  • No significant change in PMN count with antibiotic therapy

• Secondary bacterial peritonitis:

  • PMN count does not drop after 48 hours of antibiotic treatment
  • Persistent or worsening clinical symptoms despite antibiotics 1, 4

Diagnostic Algorithm

1. Initial ascitic fluid analysis: Cell count with differential, total protein, glucose, LDH
2. If lymphocytic predominance: Consider tubercular ascites and test for ADA, AFB smear, and culture
3. If neutrophilic predominance (>250 PMN/mm³):
   • Check for multiple organisms on Gram stain/culture
   • Measure glucose, LDH, total protein, CEA, and alkaline phosphatase
   • If glucose <50 mg/dL, LDH > serum upper limit, protein ≥1 g/dL, consider secondary peritonitis
   • If CEA >5 ng/mL or alkaline phosphatase >240 U/L, suspect gut perforation

Pitfalls and Caveats

• ADA may have lower sensitivity (30%) in tubercular peritonitis with underlying cirrhosis 5
• Secondary bacterial peritonitis and tubercular ascites can occasionally coexist
• Response to empiric antibiotics is crucial - lack of improvement in PMN count after 48 hours suggests secondary peritonitis 1
• Imaging studies (CT scan) are essential when secondary peritonitis is suspected 1
• Laparoscopy with peritoneal biopsy remains the gold standard for diagnosing tubercular peritonitis in uncertain cases 1