GeneXpert MTB/RIF and Ultra
Key Points
- Rapid molecular diagnostic test (results in 2 hours)
- Simultaneously detects MTB and rifampicin resistance
- Ultra version has improved sensitivity for pediatric TB
- WHO-recommended first-line test for pediatric TB
- Can be used on various specimen types
Technology Overview
Basic Principles
- Detection method:
- Real-time PCR technology
- Automated cartridge-based system
- Molecular beacon probes
- Targets rpoB gene for rifampicin resistance
- Ultra improvements:
- Larger DNA reaction chamber
- Two additional molecular targets (IS6110, IS1081)
- Optimized sample processing
- Enhanced algorithms for result interpretation
Specimen Requirements
- Suitable specimens:
- Sputum (induced/expectorated)
- Gastric aspirates
- Nasopharyngeal aspirates
- Bronchoalveolar lavage
- CSF
- Lymph node aspirates
- Pleural fluid
- Stool samples
- Sample handling:
- Minimum volume requirements
- Proper collection techniques
- Transport conditions
- Storage specifications
Performance Characteristics
GeneXpert MTB/RIF
- Sensitivity:
- Culture-positive TB: 65-75%
- Smear-positive TB: >95%
- Smear-negative TB: 55-65%
- Specificity: >98%
- RIF resistance detection: >95% accurate
GeneXpert Ultra
- Improved sensitivity:
- Culture-positive TB: 75-85%
- Smear-negative TB: 65-75%
- Paucibacillary disease: 15-20% higher
- Specificity: 95-98%
- Semi-quantitative categories:
- Trace
- Very Low
- Low
- Medium
- High
Clinical Applications
Diagnostic Algorithm
- Initial diagnostic test:
- Suspected pulmonary TB
- High-risk children
- HIV-infected children
- MDR-TB contacts
- Follow-up testing:
- Treatment monitoring
- Treatment failure evaluation
- Relapse assessment
Result Interpretation
MTB Detection Results
- Possible outcomes:
- MTB Detected
- MTB Not Detected
- Invalid/Error
- No Result
- Ultra-specific results:
- Trace calls interpretation
- Clinical correlation needed
- Previous treatment history consideration
Rifampicin Resistance Results
- Interpretation categories:
- Detected
- Not Detected
- Indeterminate
- Action points:
- Confirmation requirements
- Treatment implications
- Contact investigation needs
Special Considerations in Children
- Specimen collection challenges:
- Alternative sampling methods
- Multiple specimen testing
- Quality assurance measures
- Result interpretation nuances:
- Paucibacillary nature of disease
- Impact on sensitivity
- Clinical correlation importance
Limitations and Challenges
- Technical constraints:
- Temperature requirements
- Stable electricity needed
- Calibration requirements
- Maintenance needs
- Clinical limitations:
- False positives in previously treated patients
- Limited value in treatment monitoring
- Cost considerations
- Training requirements
Disclaimer
The notes provided on Pediatime are generated from online resources and AI sources and have been carefully checked for accuracy. However, these notes are not intended to replace standard textbooks. They are designed to serve as a quick review and revision tool for medical students and professionals, and to aid in theory exam preparation. For comprehensive learning, please refer to recommended textbooks and guidelines.
