What are the criteria for tuberculosis (TB) in ascitic fluid cell count?

Criteria for Tuberculosis in Ascitic Fluid Cell Count

Adenosine deaminase (ADA) levels ≥39 IU/L in ascitic fluid is the most reliable diagnostic criterion for tuberculous peritonitis, with sensitivity of 100% and specificity of 97%. 1

Diagnostic Approach for Tuberculous Peritonitis

Primary Diagnostic Criteria

• Adenosine Deaminase (ADA) Testing:

  • ADA level ≥39 IU/L is the optimal cut-off point for diagnosing tuberculous peritonitis 1
  • Sensitivity: 100%, Specificity: 97%
  • Different studies have shown similar cut-offs:
    • Values above 36-40 IU/L are highly suggestive of TB peritonitis 1
    • Some centers use a cut-off of 30 U/L (sensitivity 94%, specificity 92%) 2

• Cell Count Characteristics:

  • Lymphocyte predominance is typical in TB peritonitis
  • PMN count may be elevated but typically not as high as in bacterial peritonitis
  • Total white cell count is often elevated but not specific for TB

Secondary Diagnostic Features

• Ascitic Fluid Analysis:
  • Low SAAG (<1.1 g/dL) - characteristic of exudative ascites 3
  • High protein content (>2.5 g/dL) - typical of TB peritonitis
  • Acid-fast bacilli smear - rarely positive (<3% of cases)
  • Mycobacterial culture - positive in <50% of cases 3

Differential Diagnostic Considerations

1. Spontaneous Bacterial Peritonitis (SBP):

   • PMN count ≥250 cells/mm³ is diagnostic for SBP 3
   • SBP typically has neutrophil predominance rather than lymphocyte predominance
   • ADA levels are typically lower in SBP compared to TB peritonitis 4

2. Pancreatic Ascites:

   • Characterized by ascitic fluid amylase >1000 IU/L or >6× serum amylase 5
   • Protein level >3 g/dL in ascitic fluid 5
   • May also have elevated PMN count, but amylase levels differentiate it from TB

3. Malignant Ascites:

   • Cytology may be positive (variable yield)
   • ADA levels typically lower than in TB peritonitis 4

Clinical Algorithm for Diagnosing TB Peritonitis

1. Perform diagnostic paracentesis (strongly recommended for all patients with new-onset ascites) 3
2. Initial ascitic fluid analysis:
   • Total protein concentration and SAAG calculation
   • Cell count with differential
   • ADA level
   • Acid-fast bacilli smear and mycobacterial culture
3. Interpret results:
   • If ADA ≥39 IU/L: High probability of TB peritonitis
   • If ADA 30-39 IU/L: Moderate probability, consider additional testing
   • If ADA <30 IU/L: Low probability, consider alternative diagnoses

Important Caveats and Pitfalls

• False Positive ADA Results: Can occur in some cases of malignancy and empyema, though less common
• Culture Limitations: Mycobacterial culture has low sensitivity (<50%) but remains important for drug susceptibility testing 3
• Delayed Diagnosis: TB peritonitis often has an insidious onset; high clinical suspicion is needed
• Sampling Technique: Ensure adequate fluid volume (50-100 mL) for comprehensive testing including culture
• Interpretation in Cirrhosis: ADA remains useful even in the presence of underlying cirrhosis 2

Practical Recommendations

• Always send ascitic fluid for ADA testing when TB peritonitis is in the differential diagnosis
• Use flow cytometry for cell counting when available, as it provides higher specificity (100%) compared to automated cell counters (65%) for detecting elevated PMN counts 6
• Consider peritoneal biopsy if ADA results are borderline and clinical suspicion remains high
• In high TB prevalence areas, routine ADA testing should be considered for all unexplained exudative ascites 2

By following this approach, clinicians can accurately diagnose tuberculous peritonitis and initiate appropriate treatment promptly, reducing morbidity and mortality associated with delayed diagnosis.