Sputum AFB & CBNAAT (Tuberculosis / GeneXpert MTB/RIF)
Also known as: Sputum for Acid Fast Bacilli, GeneXpert MTB/RIF, CBNAAT TB Test, Tuberculosis Sputum Smear, MTB PCR
Parameters Measured in the Report
A breakdown of individual biomarkers and biological reference intervals typically found in your report:
| Biomarker | Standard Range | Unit | What It Means |
|---|---|---|---|
CBNAAT / GeneXpert MTB DNA (MTB PCR) | MTB Not Detected (Negative) | molecular PCR | Automated cartridge-based nucleic acid amplification testing (CBNAAT) detecting Mycobacterium tuberculosis complex DNA within 2 hours. |
Rifampicin Resistance (rpoB gene) (RIF Resistance) | Rifampicin Resistance Not Detected | molecular | Detects mutations in the rpoB gene conferring resistance to first-line Rifampicin; surrogate marker for Multi-Drug Resistant TB (MDR-TB). |
Sputum AFB Smear (ZN / Fluorescent) (Acid Fast Bacilli Smear) | Negative for Acid-Fast Bacilli (0 / 100 oil fields) | smear grade | Microscopic smear graded from 1+ to 3+; reflects patient infectiousness and bacterial shedding into air. |
* Note: Reference ranges may vary slightly between testing instruments and reagent kits. Always refer to the reference interval printed on your specific pathology report.
Clinical Focus
Microscopic Acid-Fast Bacilli (Ziehl-Neelsen / Auramine O fluorescent stain) and automated real-time PCR detection of Mycobacterium tuberculosis DNA plus Rifampicin resistance gene (rpoB).
Sample & Specimen
Early morning deep coughed sputum in sterile wide-mouthed container
Fasting Protocol
No fasting required. Rinse mouth with plain water before deep coughing. Do not submit saliva.
Turnaround Time
2 hours for CBNAAT (rapid molecular) | 4–8 hours for AFB smear
What Is This Test?
Why Did Your Doctor Order This Test?
Physicians routinely prescribe the TB Sputum & GeneXpert in the following clinical scenarios:
How to Prepare Before Your Test
- Sample must be true bronchial sputum (thick mucus coughed up from deep inside the lungs), NOT saliva or post-nasal drip from the mouth.
- Collect the specimen first thing in the morning upon waking: rinse mouth thoroughly with plain water to clear food debris, take three deep breaths, and cough forcefully.
- Do not use commercial chemical mouthwash before collection as it can reduce bacterial viability.
Understanding Your Results
What higher or lower readings generally mean in clinical practice:
If Values Are Higher Than Normal
A report of "MTB Detected" confirms active Pulmonary Tuberculosis, requiring immediate registration in national TB elimination programs and initiation of standard anti-tubercular therapy (HRZE regimen). If "Rifampicin Resistance Detected" is flagged, the patient has MDR-TB requiring second-line bedaquiline-based regimens.
If Values Are Lower Than Normal
A negative AFB smear and "MTB Not Detected" on CBNAAT strongly indicates that active open pulmonary TB is unlikely. If clinical suspicion remains high (e.g., cavitary lesions on CT), bronchial alveolar lavage (BAL) or liquid mycobacterial culture (MGIT) is performed.
Frequently Asked Questions
Why is saliva rejected by the laboratory for a TB test?
Saliva comes from mouth salivary glands and contains only oral bacteria, not lung pathogens. Mycobacterium tuberculosis lives deep inside lung alveoli and bronchial walls. Only true sputum (thick mucus coughed up from the chest) contains diagnostic TB bacilli.
What is the advantage of GeneXpert CBNAAT over traditional sputum microscopy?
Microscopy requires at least 5,000 to 10,000 bacteria per milliliter of sputum to be visible under a lens. GeneXpert PCR detects as few as 100 bacteria per milliliter and determines within 2 hours if the patient is resistant to Rifampicin.
Related Laboratory Tests
Tests frequently prescribed together or used as follow-up investigations:
Complete Blood Count (CBC / Hemogram)
Counts and evaluates red blood cells, white blood cells, hemoglobin, hematocrit, and platelets.
Inflammatory Markers (ESR & C-Reactive Protein / hs-CRP)
Rate of red blood cell settling (ESR) and hepatic acute-phase reactant protein concentration (CRP / hs-CRP).
Liver Function Test (LFT / Hepatic Panel)
Bilirubin fractions, transaminases (ALT/SGPT, AST/SGOT), Alkaline Phosphatase (ALP), and proteins (Albumin, Globulin).
Medical & Diagnostic Disclaimer
This guide is published for educational and patient informational purposes only. It is not a substitute for professional medical evaluation, diagnosis, or clinical advice.
A laboratory result that falls slightly outside standard statistical reference intervals does not automatically indicate disease; conversely, a normal result does not guarantee the absence of medical conditions. Only your attending physician can interpret lab findings in the context of your medical history, physical exam, and symptoms.