AFB Smear in Tuberculous Peritonitis: Limited Diagnostic Value
AFB smear microscopy should still be performed on peritoneal fluid despite its extremely poor sensitivity (0-86%) because a positive result has high specificity (≥90%) and can guide immediate treatment decisions, but a negative result cannot exclude tuberculosis peritonitis. 1
Diagnostic Performance of AFB Smear in Peritoneal Fluid
Sensitivity and Specificity
- AFB smear has dismal sensitivity ranging from 0-86% in peritoneal fluid, making it one of the least sensitive tests for tuberculous peritonitis 1, 2, 3
- However, specificity remains ≥90%, meaning false-positive results are unlikely 1
- This paucibacillary nature of extrapulmonary TB explains why AFB smear performs so poorly compared to pulmonary TB 1
Clinical Interpretation Algorithm
- If AFB smear is positive: Proceed with antituberculosis treatment immediately, as false-positives are rare and this confirms the diagnosis 1
- If AFB smear is negative: Do NOT use this to exclude tuberculous peritonitis—false-negatives are exceedingly common and treatment decisions must rely on other diagnostic modalities 1
Superior Diagnostic Alternatives in Your Clinical Scenario
ADA Levels: The Preferred Test
Given your patient already has elevated ADA levels, this is far more diagnostically useful than AFB smear:
- ADA ≥32-40 U/L in non-cirrhotic patients demonstrates 100% sensitivity and 96.6-100% specificity for tuberculous peritonitis 2, 4
- ADA ≥27-32 U/L in cirrhotic patients maintains 91.7-100% sensitivity and 92-93.3% specificity 2, 4
- Meta-analysis shows ADA in peritoneal fluid has 100% sensitivity and 97% specificity at appropriate thresholds 1
Mycobacterial Culture: Better Than AFB Smear
- Culture sensitivity ranges 45-69% in peritoneal fluid—substantially better than AFB smear 1
- Culture specificity exceeds 97% 1
- Strong recommendation: Mycobacterial cultures should always be performed on peritoneal fluid specimens 1
Critical Clinical Pitfall to Avoid
Do not delay antituberculosis treatment while awaiting AFB smear, culture, or other microbiologic confirmation. 2 The combination of:
- Clinical presentation (ascites, abdominal pain)
- Elevated ADA levels (≥32-40 U/L)
- Exclusion of malignancy, uremia, trauma, and bacterial peritonitis
...is sufficient to initiate treatment, as culture sensitivity is only 20-83% and AFB smear is even worse at 0-86%. 2, 3
Practical Diagnostic Approach
Order AFB smear alongside these tests:
- Mycobacterial culture (mandatory—higher yield than smear) 1
- Cell count with differential (lymphocytic predominance suggests TB) 3
- Total protein (typically >2.5 g/dL in TB peritonitis) 5
- Glucose (may be low) 3
In endemic areas with elevated ADA: Start empiric antituberculosis treatment after excluding malignancy, uremia, trauma, and bacterial peritonitis, regardless of AFB smear results 2
In non-endemic areas (like the United States): Exercise more caution, as ADA sensitivity drops to 58.8% overall and only 30% in cirrhotic patients, with occasional false-positives from malignancy (13%) and bacterial peritonitis (5.8%) 5
Why AFB Smear Still Has a Role
Despite poor sensitivity, the ATS/IDSA/CDC guidelines conditionally recommend performing AFB smear because: 1
- It provides opportunity for early diagnosis when positive
- The high specificity means a positive result can guide immediate treatment decisions
- It's rapid, inexpensive, and widely available
- The conditional recommendation acknowledges the very low-quality evidence but recognizes clinical utility when positive
The key is understanding that AFB smear is a "rule-in" test (positive = treat) but never a "rule-out" test (negative = meaningless) for tuberculous peritonitis. 1