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🧬 What is GeneXpert?

GeneXpert (also known as CBNAAT – Cartridge-Based Nucleic Acid Amplification Test) is a rapid molecular diagnostic test that detects the DNA of Mycobacterium tuberculosis (MTB) and simultaneously identifies rifampicin resistance, a key marker for multidrug-resistant TB (MDR-TB).

Platform: Cepheid GeneXpert system
Method: Real-time PCR (Polymerase Chain Reaction) using a cartridge-based system
Sample types: Primarily sputum, but also CSF, gastric aspirate, lymph node aspirates, tissue, pleural/peritoneal fluid, etc.

❓ Why is the Test Done?

To:

Rapidly diagnose tuberculosis (TB) (pulmonary or extrapulmonary)
Detect rifampicin resistance, which suggests MDR-TB
Guide early initiation of appropriate anti-TB therapy
Evaluate suspected TB in high-risk populations, including:
- HIV-positive patients
- Children
- Extrapulmonary TB suspects
- Previously treated TB patients

📊 Normal Range / Reference

Result	Interpretation
MTB Not Detected	No Mycobacterium tuberculosis DNA found
MTB Detected	DNA of TB bacteria found; TB infection confirmed
Rifampicin Resistance – Not Detected	TB is likely drug-sensitive
Rifampicin Resistance – Detected	Indicates rifampicin-resistant TB (possible MDR-TB)
Indeterminate	Invalid or inconclusive; sample may need to be repeated

📈 Interpretation of Results

Result	Clinical Action
MTB detected + No Rif resistance	Start first-line anti-TB drugs (HRZE)
MTB detected + Rif resistance	Treat as MDR-TB; refer for Line Probe Assay (LPA) and culture sensitivity
MTB not detected	TB unlikely; consider alternative diagnoses or repeat with better sample
Indeterminate	Repeat test or confirm with culture or AFB smear

🧠 Associated Conditions and Sample Types

Condition	Sample
Pulmonary TB	Sputum, BAL, endotracheal aspirate
TB meningitis	CSF
Abdominal TB	Ascitic fluid, peritoneal tissue
Lymph node TB	Fine needle aspirate (FNA) or biopsy
Bone/joint TB	Tissue biopsy, synovial fluid

🔄 Related / Follow-Up Tests

AFB smear microscopy
TB Culture (MGIT/Bactec) – gold standard but slower
Line Probe Assay (LPA) – for resistance to INH, Rif, and second-line drugs
Histopathology – for granulomas
Chest X-ray / CT – assess pulmonary TB
HIV test – co-testing recommended

✅ Fasting Required?

Parameter	Requirement
GeneXpert (CBNAAT)	❌ No fasting required – Sample timing depends on site of infection (e.g., early morning sputum is preferred for pulmonary TB)

📝 Summary Table

Parameter	Details
What	Rapid PCR-based test to detect Mycobacterium tuberculosis DNA and rifampicin resistance
Why	To confirm TB and assess drug resistance quickly
Normal Result	MTB Not Detected
Abnormal Result	MTB Detected ± Rifampicin Resistance
Associated Conditions	Pulmonary TB, lymph node TB, TB meningitis, abdominal TB
Follow-up Tests	AFB smear, culture, LPA, HIV test
Fasting Required	❌ No

Genxpert

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