AFB-Xpert panel (M.Tb Detection & Rifamipicin resistance) by CBNAAT - Pulmonary samples

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WHO-endorsed rapid diagnostic test for tuberculosis (TB), especially extrapulmonary TB where traditional methods often fail.

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AFB-Xpert Panel (M.Tb Detection & Rifamipicin Resistance) by CBNAAT - Pulmonary Samples

Why is it done?
- Rapid detection of Mycobacterium tuberculosis (M.Tb) in respiratory samples using molecular technology (Xpert MTB/RIF assay)
- Simultaneous detection of rifampicin resistance, which indicates multidrug-resistant tuberculosis (MDR-TB)
- Primary screening test for tuberculosis in patients with respiratory symptoms (cough ≥2 weeks, fever, night sweats, weight loss)
- Initial diagnosis of pulmonary TB in treatment-naive patients and those with previous TB history
- Early identification of drug-resistant TB strains to enable prompt initiation of appropriate anti-TB therapy
- Preferred test for immunocompromised patients (HIV/AIDS) with suspected TB
- Contact tracing in individuals exposed to confirmed TB cases
- WHO-recommended initial diagnostic test, especially in resource-limited settings
Normal Range
- Normal/Negative Result: M.Tb NOT DETECTED and Rifampicin Resistance NOT DETECTED
- Positive Result Categories:
- • M.Tb DETECTED; Rifampicin Susceptible (RIF S): Tuberculosis present; patient is sensitive to standard first-line therapy
- • M.Tb DETECTED; Rifampicin Resistant (RIF R): Tuberculosis present with MDR-TB or XDR-TB; requires second-line anti-TB drugs
- Indeterminate Result: Test quality issue; repeat testing recommended
Interpretation
- Negative Result (M.Tb NOT DETECTED):
  - Indicates absence of active pulmonary TB in the specimen tested
  - Does not completely rule out TB; consider clinical context, X-ray findings, and repeat testing if suspicion remains high
  - Sensitivity of CBNAAT: 95-98% for smear-positive TB; 87-92% for smear-negative TB
- Positive Result - M.Tb DETECTED; Rifampicin Susceptible (RIF S):
  - Confirms active pulmonary tuberculosis infection
  - Patient is sensitive to rifampicin and can be treated with standard first-line anti-TB regimen (HRZE: Isoniazid, Rifampicin, Pyrazinamide, Ethambutol)
  - Requires confirmation by culture for drug susceptibility testing (DST) in some settings
  - Specificity of CBNAAT: >99% for TB diagnosis
- Positive Result - M.Tb DETECTED; Rifampicin Resistant (RIF R):
  - Confirms active pulmonary tuberculosis with rifampicin resistance
  - Indicates likely multidrug-resistant tuberculosis (MDR-TB); resistance to both isoniazid and rifampicin is implied
  - Requires urgent initiation of second-line anti-TB therapy (fluoroquinolones + injectable agents + newer agents)
  - Requires confirmation by culture and comprehensive DST for all second-line drugs
  - Patient is at higher risk for treatment failure and poor outcomes
- Factors Affecting Test Accuracy:
  - Quality of specimen collection (inadequate or contaminated samples reduce sensitivity)
  - Bacterial load in specimen (lower sensitivity in smear-negative cases)
  - Sample type (sputum most sensitive; less sensitive with bronchoalveolar lavage or gastric washings)
  - Timing of specimen collection (early morning sputum preferred)
  - Stage of disease (higher sensitivity in active TB with high bacillary load)
Associated Organs
- Primary Organ System:
  - Respiratory system (lungs, bronchi, trachea)
- Diseases and Conditions Diagnosed:
  - Pulmonary tuberculosis (active disease)
  - Multidrug-resistant tuberculosis (MDR-TB)
  - Extensively drug-resistant tuberculosis (XDR-TB)
- Potential Complications of Abnormal Results:
  - If untreated TB: progressive lung destruction, cavitary disease, hemoptysis, respiratory failure
  - Disseminated TB: miliary TB, TB meningitis, TB of bones/joints, TB lymphadenitis
  - MDR-TB: treatment-resistant disease, higher mortality, increased healthcare costs, prolonged infectivity
  - Secondary bacterial infections of damaged lungs
  - Chronic lung fibrosis and obstructive airway disease
  - Transmission risk to close contacts (family, healthcare workers, community members)
Follow-up Tests
- For Positive Results (M.Tb DETECTED):
  - Mycobacterial culture from sputum/respiratory samples (gold standard for TB diagnosis; enables full drug susceptibility testing)
  - Conventional drug susceptibility testing (DST) for isoniazid, streptomycin, ethambutol, and all second-line drugs
  - Liquid culture systems (MGIT) for faster culture confirmation (2-4 weeks vs 4-8 weeks on solid media)
  - Chest X-ray to assess extent of disease, cavitary changes, and for screening of contacts
  - HIV testing to identify co-infection status and guide treatment planning
  - Baseline liver and renal function tests (AST, ALT, creatinine) before starting anti-TB therapy
  - Audiometry before starting treatment (for monitoring aminoglycoside toxicity if XDR-TB)
- For Rifampicin Resistant Results:
  - Urgent culture and DST for comprehensive evaluation of resistance profile (MDR-TB vs XDR-TB)
  - Genotypic testing (e.g., Xpert MTB/RIF assay on repeat sample, line probe assay, or whole-genome sequencing) for rapid confirmation
  - Repeated Xpert MTB/RIF on high-probability patients (to confirm resistance if initial test showed RIF R)
  - DST for all second-line TB drugs (fluoroquinolones, injectable agents, linezolid, bedaquiline, etc.)
- For Negative Results:
  - Repeat Xpert MTB/RIF on different days if clinical suspicion remains high (sensitivity improves with multiple samples)
  - AFB smear microscopy on follow-up samples
  - Mycobacterial culture if clinical suspicion remains high
  - Investigate alternative diagnoses (fungal infections, atypical infections, malignancy, non-TB respiratory conditions)
- During Treatment Monitoring:
  - Sputum microscopy or Xpert MTB/RIF at 2 months and 5-6 months of therapy to assess treatment response
  - Routine liver and renal function tests during therapy (especially months 1, 3, and 6)
  - Repeat Xpert MTB/RIF if there is no smear/culture conversion after 2 months of therapy
  - End-of-treatment sputum examination to confirm treatment completion and non-infectivity
Fasting Required?
- Fasting: NO
  - This is a respiratory sample test (sputum collection) and does not require fasting
- Special Specimen Collection Instructions:
  - Preferably collect early morning first sputum sample (highest bacillary load after overnight accumulation)
  - Patient should rinse mouth with water (not mouthwash) before sample collection
  - Collect at least 5-10 mL of sputum in a sterile, leak-proof container
  - Ensure sample is genuinely sputum (from lower respiratory tract), not saliva
  - Collect on consecutive days for optimal detection (preferably 2-3 samples)
  - Handle samples with appropriate biosafety precautions (category 3 pathogen)
  - Process samples as soon as possible (within 2-4 hours; refrigerate if delay is necessary)
- Medications to Avoid:
  - NO medications need to be avoided prior to specimen collection
  - However, avoid use of cough suppressants or expectorants for at least 30 minutes before collection to ensure natural sputum production
- Additional Patient Preparation:
  - Patient education on proper sputum collection technique is crucial for obtaining valid samples
  - Inform patients about biosafety precautions and need for infection control during sample collection
  - No specific diet or activity restrictions are required

How our test process works!