Mycobacterium tuberculosis detection

Scope

TB detection spans latent infection screening (interferon-gamma release assay or tuberculin skin test - detecting immune sensitisation without active disease) and active disease confirmation (sputum/tissue microscopy for acid-fast bacilli, culture, and nucleic acid amplification testing) - these answer fundamentally different clinical questions and are not interchangeable
TB remains a major global cause of infectious disease mortality, with detection strategy heavily influenced by whether the clinical question is screening (asymptomatic, high-risk/pre-immunosuppression populations) or diagnosing symptomatic active disease

Interferon-gamma release assays (IGRA) measure T-cell response to specific M. tuberculosis antigens, offering better specificity than tuberculin skin testing (less cross-reactivity with BCG vaccination) but cannot distinguish latent from active disease - a positive result in a symptomatic patient still requires active disease investigation
Acid-fast bacilli (AFB) smear microscopy gives a rapid presumptive result but lower sensitivity than culture; culture remains the reference standard (also enabling drug susceptibility testing) but is slow (weeks) given the organism's slow growth; nucleic acid amplification (e.g. GeneXpert) provides rapid results with good sensitivity and can detect rifampicin resistance markers

Active disease work-up: sputum (or other relevant site) for AFB smear, culture, and NAAT, alongside chest imaging; multiple samples improve sensitivity
Latent infection screening (IGRA/TST) is used in specific contexts - contact tracing, pre-immunosuppression screening, and high-risk population screening - not as a routine active-disease test

Active TB: multi-drug combination therapy for a prolonged course (standard regimens typically 6 months for drug-susceptible disease), with drug susceptibility results guiding regimen adjustment for resistant disease
Latent TB (positive IGRA/TST without active disease): treatment considered based on risk of reactivation (immunosuppression risk, recent conversion, close contact exposure) to prevent progression to active disease

Drug-resistant TB (multidrug-resistant, extensively drug-resistant - a growing global concern), latent TB screening before biologic/immunosuppressive therapy, BCG vaccination effect on tuberculin skin testing (not on IGRA)

A positive IGRA or tuberculin skin test cannot distinguish latent infection from active disease - a symptomatic patient with a positive screening test still requires full active-disease investigation (imaging, sputum sampling) rather than being managed as latent infection based on the screening result alone

5 more sections, plus exam facts

Premium unlocks every note across every specialty, and the full exam fact library behind it.