Specimen Medium for Colonic TB GeneXpert Testing
For suspected colonic tuberculosis, obtain tissue biopsies from colonoscopy (including normal-appearing mucosa) and submit them directly in sterile saline or dry container for GeneXpert MTB/RIF testing. 1, 2
Optimal Specimen Type and Collection
Colonoscopic biopsies are the preferred specimen for GeneXpert testing in colonic TB, with demonstrated sensitivity of 95.7% when performed on gastrointestinal tissue samples. 2
Multiple biopsies should be obtained from both visibly abnormal areas and normal-appearing terminal ileum and cecum, as histologically confirmed TB can be present even when mucosa appears endoscopically normal (diagnostic yield of 8% from normal-appearing tissue). 3
Fresh tissue specimens should be collected and can be processed after grinding in a sterile mortar before GeneXpert testing. 1
Specimen Processing and Submission
No special transport medium is required - tissue biopsies can be submitted in sterile saline or dry containers, then processed by grinding/crushing and sonication before GeneXpert analysis. 1
Reserve sufficient tissue for both GeneXpert testing AND conventional culture (Lowenstein-Jensen solid medium and MGIT liquid medium), as culture remains the gold standard and is required for drug susceptibility testing. 4, 1
Stool specimens are an alternative when tissue biopsy is not feasible or yields insufficient material, with GeneXpert sensitivity of 39.1% for intestinal TB (though significantly lower than tissue biopsy). 5
Critical Performance Characteristics
GeneXpert on gastrointestinal biopsies vastly outperforms culture (95.7% sensitivity versus 35.0% for culture against composite gold standard), making it the superior rapid diagnostic test for colonic TB. 2
Tissue GeneXpert is far superior to stool GeneXpert for intestinal TB diagnosis - tissue sensitivity 95.7% versus stool sensitivity 39.1%, with tissue specificity also higher at 96.2% versus 85.7%. 5, 2
Histopathology alone is inadequate - in documented cases of colonic TB confirmed by GeneXpert and culture, histology showed only "non-specific chronic colitis" without granulomas or other TB features. 1
Important Clinical Caveats
Negative histology does not exclude colonic TB - molecular testing with GeneXpert can be positive even when histological examination shows no granulomas or specific TB features. 1
Always obtain biopsies from terminal ileum when possible (achievable in 86% of colonoscopies), as ileocecal involvement is common and terminal ileum biopsies may be diagnostic even when cecal biopsies are negative. 3
Results should be available within 48 hours of specimen collection to enable rapid treatment initiation and improved patient outcomes. 4, 6
Culture must still be performed alongside GeneXpert to enable drug susceptibility testing and to detect rifampin resistance, which GeneXpert can identify but culture confirms. 4, 1