Mycobacterium tuberculosis Laboratory Diagnosis
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Direct answer
Mycobacterium tuberculosis laboratory diagnosis rests on sputum microscopy for acid-fast bacilli, culture on solid or liquid media, and molecular tests such as CBNAAT that detect the organism and rifampicin resistance together. Culture remains the reference method for confirmation and full drug susceptibility testing, although it is slow. Under the national programme, CBNAAT is the initial test for presumptive pulmonary disease.
What you must remember
- Ziehl-Neelsen stain: carbolfuchsin is retained by the mycolic acid-rich cell wall after acid-alcohol decolourisation, so bacilli stay red against a methylene blue background.
- Smear threshold: microscopy shows bacilli only when roughly 10,000 bacilli per millilitre of sputum are present, so a negative smear does not exclude disease.
- LED microscopy: fluorescence microscopy with auramine-O screens slides faster than Ziehl-Neelsen light microscopy and is used for high-volume screening.
- Lowenstein-Jensen medium: an egg-based solid medium on which Mycobacterium tuberculosis grows over several weeks, producing buff, rough colonies.
- MGIT culture: a liquid system that detects growth sooner than solid media and is used for drug susceptibility testing.
- CBNAAT: Xpert MTB/RIF detects Mycobacterium tuberculosis DNA and rifampicin resistance through the rpoB gene in about two hours.
- Line probe assay: reverse hybridisation strips detect rpoB, katG and inhA mutations, and second-line versions detect fluoroquinolone and injectable resistance.
A diagnostic pathway for presumptive pulmonary tuberculosis
A 35-year-old has a cough for three weeks, night sweats and weight loss. The steps set out how specimens and tests are sequenced in a national programme.
- Collect good specimens: sputum is collected in a sterile container, with an early morning sample preferred, while extrapulmonary disease needs biopsy, fluid or pus as appropriate.
- Run CBNAAT first: Xpert MTB/RIF is the initial molecular test for presumptive tuberculosis, and a rifampicin-resistant result triggers the drug-resistant pathway.
- Add a smear as support: Ziehl-Neelsen or LED microscopy is quick and indicates infectiousness, though a negative smear never excludes tuberculosis.
- Send culture for confirmation: Lowenstein-Jensen or MGIT culture grows the organism slowly and remains essential for drug susceptibility testing across all drugs.
- Test resistance rapidly: a line probe assay on smear-positive samples or culture isolates identifies isoniazid and rifampicin mutations within days, guiding the regimen.
- Report and act: results reach the treating clinician and the programme, so that rifampicin-resistant or isoniazid-resistant patients are referred for appropriate treatment.
Where the exam tries to catch you
Diagnosis questions test which method answers which question: presence, resistance, or viable isolate.
- Trap: believing a negative smear rules out tuberculosis Correct: smear is insensitive, so a negative smear with a positive culture or CBNAAT is common in paucibacillary disease.
- Trap: choosing Lowenstein-Jensen as the fastest confirmation Correct: LJ is slow, and MGIT and CBNAAT deliver results much sooner.
- Trap: assuming CBNAAT gives full drug susceptibility Correct: it detects rifampicin resistance only, so isoniazid and other drugs still need line probe assay or phenotypic testing.
Frequently asked questions
Why is Ziehl-Neelsen staining used for Mycobacterium tuberculosis?
Ziehl-Neelsen staining uses carbolfuchsin, which the mycolic acid-rich cell wall retains even after acid-alcohol decolourisation. Acid-fast bacilli therefore appear red against a blue counterstain, which makes smear microscopy quick and inexpensive.
What does CBNAAT detect in tuberculosis diagnosis?
CBNAAT, based on Xpert MTB/RIF, detects Mycobacterium tuberculosis DNA and resistance to rifampicin through mutations in the rpoB gene. It gives results in about two hours but does not provide susceptibility results for isoniazid or other drugs.
Why is culture still needed when CBNAAT is available?
Culture produces a viable isolate for drug susceptibility testing across all drugs, which molecular assays cannot fully replace. It also confirms the diagnosis when smear and molecular results are inconclusive.
What is the difference between Lowenstein-Jensen and MGIT culture?
Lowenstein-Jensen is a solid egg-based medium on which Mycobacterium tuberculosis grows over several weeks, whereas MGIT is a liquid culture system that detects growth sooner. MGIT is therefore preferred when faster drug susceptibility results are needed.
What does the line probe assay detect in Mycobacterium tuberculosis?
The line probe assay detects mutations that cause resistance to isoniazid and rifampicin, such as those in the katG, inhA and rpoB genes. Second-line versions detect fluoroquinolone and injectable resistance, which informs the choice of regimen.