Expansion
Smear, culture and NAAT for active disease; Mantoux and IGRA for latent
Expansion
Active disease
- Smear microscopy: Ziehl-Neelsen or auramine for acid fast bacilli. Fast and cheap, but insensitive, requiring around 10,000 organisms per millilitre, and it does not distinguish M. tuberculosis from non-tuberculous mycobacteria. Three sputum samples, including one early morning
- Culture: the reference standard, giving speciation and full drug sensitivities, but takes 2 to 6 weeks on liquid media and up to 8 on solid
- Nucleic acid amplification (NAAT): results in hours, confirms M. tuberculosis and detects rifampicin resistance. Used on smear positive samples and where a rapid answer changes management, such as suspected meningitis or drug resistance
- Histology: caseating granulomas on biopsy
- Imaging: upper lobe cavitation, hilar lymphadenopathy, miliary pattern
Latent infection
- Mantoux (tuberculin skin test): 0.1 ml of purified protein derivative intradermally, read at 48 to 72 hours by measuring induration, not erythema. False positive with prior BCG and non-tuberculous mycobacteria; false negative in immunosuppression, HIV, miliary or overwhelming disease, recent viral infection, recent live vaccine and extremes of age
- Interferon gamma release assay: a blood test, unaffected by BCG, requiring only a single visit, but more expensive and with indeterminate results in the immunosuppressed
Neither latent test excludes active disease, and both are negative in a proportion of proven active cases, so a negative result must never override clinical and radiological suspicion.
Extrapulmonary samples: send fluid in a large volume for culture, since yields are low, and always request mycobacterial culture specifically, since it is not performed routinely.