Infection: Tuberculosis; Mycobacterium tuberculosis; TB

Brief description:
  • Lung infection is the most common presentation, however extra-pulmonary disease is also common. TB can involve essentially any body system.
  • TB is considered potentially infectious if the lungs are involved. Extra-pulmonary infection is not infectious, unless the infection communicates with the outside world.
  • Consider TB in people with risk factors (born in high incidence country or prior exposure).
Did you know?

In general, TB is a slow moving disease. Symptoms will often be gradual in onset or prolonged. People presenting with an acute illness are less likely to have TB as the cause.

Test of choice and who should I test?

Request Mycobacterial culture

Culture is the gold standard diagnostic test. This allows full identification and susceptibility testing of the organism. Sample type will depend on the affected tissue or organ.

  • Pulmonary disease: sputum (induced/bronchoscopy sample if non-productive)
  • Extra-pulmonary: biopsy or aspirate of affected tissue e.g. lymph node biopsy, pleural fluid aspirate, urine for renal TB
  • Excellent specificity – a positive culture confirms the diagnosis
  • Variable sensitivity – depends on the sample type
  • TB doesn’t grow on normal culture media, so TB/mycobacterial culture needs to be specifically requested on the form. Typically takes 2-3 weeks to come positive.

AFB microscopy is automatically done by the lab if TB/mycobacterial culture is requested

  • Used to help assess the infectiousness of people with pulmonary TB.
  • It has limited sensitivity and specificity as a stand-alone diagnostic test for TB.

PCR (e.g. GeneXpert) can also be used to diagnose TB

  • Sensitivity is slightly lower than culture, but specificity is similarly high.
  • Can rule in the diagnosis rapidly compared to culture, but does not provide full susceptibility information to guide treatment – culture is still required.
  • Mostly used if a rapid result is particularly important e.g. unwell patient who needs urgent treatment, significant Public Health implications.
Tests to avoid/specialist tests:

QuantiFERON-TB Gold is a test for past exposure to TB (latent infection)

  • Is used in people who may benefit from latent TB treatment e.g. prior to certain types of immunosuppression, immigrants from high incidence countries.
  • Is not recommended to investigate for possible active TB, as it can neither rule it in nor rule it out. It is occasionally used by specialists if a culture-based diagnosis is difficult, however it cannot provide a definitive answer.
Other considerations:
  • TB is a notifiable disease. Seek specialist help urgently if pulmonary TB is suspected, as it may be a Public Health risk.
  • Taking samples, e.g. sputum, biopsies, can potentially expose the collector to infectious aerosols. Wear appropriate PPE if TB is a possibility (i.e N95, or equivalent, mask).