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Diagnosis relies on capturing bacilli from the lower respiratory tract. Saliva (secreted by the submandibular/parotid glands) lacks the cellular material and pathogen load found in endobronchial secretions.
Pro-tip: For patients who cannot expectorate, gastric lavage (especially in peds) or induced sputum (using hypertonic saline) are the standard alternatives.
Auramine-O (sometimes with Rhodamine) binds to the mycolic acids in the cell wall. Under a fluorescent microscope, the bacilli appear as bright yellow-orange rods against a dark background.
ZN smears require 1000x (oil immersion) and at least 100 fields for a "negative" report. LED-FM can be screened at 200x or 400x. This allows a technician to cover the same area in a fraction of the time, significantly reducing "eye fatigue" in high-volume labs like yours.
It uses semi-nested real-time PCR to target the rpoB gene. It doesn't just say "TB is there"; it tells you if the rpoB gene has mutations that signify Rifampicin resistance, which is a proxy for MDR-TB.
Unlike CBNAAT, which looks for a "missing" wild-type signal, LPA uses hybridization to identify specific mutations (like katG or inhA for Isoniazid, and gyrA for fluoroquinolones). It’s the backbone of detecting XDR-TB patterns.
Room temperature allows commensal flora to overgrow, which can "contaminate" the culture (even after decontamination with NALC-NaOH). Cold chain (2-8 C) keeps the flora in check.
3–5 mL is the "ideal." amount. Less than that reduces the sensitivity of smear and culture; more than that doesn't significantly increase yield but increases the work for decontamination.
While molecular tests are faster, culture remains the gold standard because it confirms viability and allows for phenotypic DST, which is still the definitive way to determine exactly which drugs will kill that specific strain.
This follows WHO/RNTCP (NTEP) guidelines for Category B infectious substances.
Primary: Leak-proof specimen container.
Secondary: Leak-proof, sieved/cushioned container.
Absorbent: Must be enough to soak up the entire volume of the liquid if the primary container breaks.
This follows WHO/RNTCP (NTEP) guidelines for Category B infectious substances.
Primary: Leak-proof specimen container.
Secondary: Leak-proof, sieved/cushioned container.
Absorbent: Must be enough to soak up the entire volume of the liquid if the primary container breaks.
The reagent (containing NaOH and isopropanol) liquefies sputum, reduces viability of bacilli, and releases DNA for amplification.
Results are typically available within 90–120 minutes.
False negatives may occur in low bacillary load cases; clinical correlation and further testing may be required.
The assay detects mutations in rpoB gene associated with rifampicin resistance.
Errors can result from cartridge issues, power failure, improper sample preparation, or PCR inhibition.
CBNAAT is a cartridge-based nucleic acid amplification test that uses real-time PCR to detect Mycobacterium tuberculosis complex DNA and rifampicin resistance.
Mutations in the 81-bp rifampicin resistance-determining region (RRDR) of the rpoB gene are detected.
CBNAAT can detect rifampicin resistance along with MTB, which smear microscopy cannot.
Rifampicin resistance is considered a surrogate marker for MDR-TB (since INH resistance often coexists).
CBNAAT detects DNA; it cannot distinguish between viable and non-viable organisms
Tissues - Practice Quiz
Test your knowledge about the 5 seeing habits discussed in the text 'UPS 5 seeing habits'.