Xpert MTB/RIF Specimen Recommendations
For pulmonary tuberculosis diagnosis, respiratory specimens—specifically sputum (expectorated or induced), bronchoalveolar lavage fluid (BALF), bronchial aspirates, or bronchial wash—are the recommended specimens for Xpert MTB/RIF testing. 1
Primary Specimen Types by Clinical Presentation
Pulmonary Tuberculosis
- Expectorated or induced sputum is the first-line specimen, with at least 2 specimens collected for microscopy and 1 for Xpert MTB/RIF testing 2
- The assay demonstrates 85% pooled sensitivity and 98% specificity on respiratory specimens 3
- In AFB smear-positive respiratory specimens, sensitivity reaches 96-97% with specificity of 85-95% 1
- In AFB smear-negative respiratory specimens, sensitivity drops to 66-79% but specificity remains high at 80-98% 1
When Sputum Cannot Be Obtained
- Bronchoalveolar lavage fluid (BALF) serves as an excellent alternative when patients cannot produce adequate sputum 4, 5
- Bronchial aspirates and bronchial wash specimens are also acceptable, with BALF showing 31.4% positivity compared to 12.1% for bronchial wash in one study 5
- The Xpert assay on BALF demonstrates superior accuracy to smear microscopy and powerful capability to identify cases undetected by conventional methods 4
Pediatric Considerations
- Children frequently cannot produce sputum, but induced sputum should be attempted first 6
- Stool specimens can be considered in children as an alternative, though sensitivity is substantially lower at 55.6% compared to sputum 6
- Gastric aspirates are another option in pediatric patients unable to produce sputum 7
Extrapulmonary Tuberculosis Specimens
Lymph Node Specimens
- Lymph node aspirates or tissue show the highest sensitivity among extrapulmonary specimens at 90% 8
- Xpert MTB/RIF may be used as the initial diagnostic tool for TB lymphadenitis 8
Body Fluids with Limited Utility
- Pleural fluid shows poor sensitivity at only 30%, limiting its diagnostic value 8
- Peritoneal fluid similarly demonstrates low sensitivity at 32% 8
- Cerebrospinal fluid shows moderate sensitivity at 53% 8
- Despite lower sensitivity, specificity remains high at 98% across all extrapulmonary specimens 8
Critical Implementation Points
Specimen Handling
- Sufficient specimen volume must be reserved for culture before performing Xpert MTB/RIF, as culture remains the gold standard regardless of NAAT results 1
- Both liquid and solid cultures must always be performed on all specimens 1
- Results should be available within 48 hours of specimen collection 1
Common Pitfall to Avoid
- Never rely on Xpert MTB/RIF alone for extrapulmonary specimens (except lymph node tissue), as the sensitivity is inadequate for ruling out disease 8
- The presence of inhibitors can cause false-negative results; if AFB smear is positive but Xpert is negative, evaluate for inhibitors and consider collecting a new specimen 9
- A single negative Xpert result cannot definitively exclude tuberculosis, especially with moderate-to-high clinical suspicion 1