Clinical use
Investigation of samples for the presence of Mycobacterium species.
Background
TB is caused by members of the Mycobacterium tuberculosis complex (MTBC); in humans this is predominantly Mycobacterium tuberculosis, though less often by other members of the complex such as Mycobacterium bovis, Mycobacterium africanum, Mycobacterium canettii or Mycobacterium caprae.
Strains of the live vaccine Mycobacterium bovis BCG, which are also used intra-vesically in the treatment of bladder cancer, can occasionally cause disease in patients who are immunocompromised. The two other species currently in the complex, Mycobacterium microti and Mycobacterium pinnipedii are almost exclusively associated with different mammalian hosts, but a few cases have been reported in patients who are immunocompromised.
Non-tuberculous mycobacteria (NTM) are also increasingly encountered as a cause of disease in humans, especially in higher risk groups such as those with cystic fibrosis, chronic obstructive pulmonary disease, bronchiectasis and immunosuppression
Not all persons infected with tubercle bacilli develop disease, and not all those that are infected become infectious to others. Overt disease may develop months or years after the initial exposure. Patients with evidence of potential acquisition of MTBC, for example, on the basis of a positive tuberculin skin test and/or an interferon gamma blood test, but with no symptoms of active disease nor positive sample for MTBC may have latent TB infection (LTBI). Disease due to tuberculosis may occur in virtually any organ of the body but is most common in the lungs and infection is characterized by caseating granuloma formation.
TB is a notifiable disease, and clinically suspected cases should be notified to the local Health Protection team by the treating clinician. All cases of TB should be tested for HIV.
Patient preparation
Collect specimens before antimicrobial therapy where possible.
Some antimicrobials may also have significant anti-mycobacterial activity, notably the fluoroquinolones such as ciprofloxacin, levofloxacin or moxifloxacin, and the macrolides such as clarithromycin or azithromycin.
Specimen requirements
Send in a sterile container
Where possible ensure all samples are fresh (less than 1 day old) to minimise contamination. Samples received in formalin will not be processed.
Send all samples investigated for Mycobacteria with the appropriate yellow ‘Danger of Infection’ sticker, these samples pose a risk to laboratory staff handling them and are therefore processed under different conditions to other samples. If a sticker is not available at requesting sites, please write ‘Danger of Infection’ on the request form.
Sputum
- Minimum volume: >5ml
- Storage and transport: Refrigerate if transport to the laboratory will be delayed by more than 1 hour
Notes
- Send 2 to 3 samples each 8-24 hours apart, to increase diagnostic yield
- One sample should be expectorated early morning
- Purulent samples are preferred, clear saliva is not recommended
Bronchoalveolar lavage
- Minimum volume: >5ml
- Storage and transport: Refrigerate if transport to the laboratory will be delayed by more than 1 hour
Notes
Contamination of the bronchoscope with tap water, which may contain environmental Mycobacterium species, should be avoided
Pleural/pericardial/joint fluid
- Minimum volume: >5ml
- Storage and transport: Refrigerate if transport to the laboratory will be delayed by more than 1 hour
Notes
- Poor sensitivity, consider sending concurrent tissue biopsy sample
- Negative result does not rule out the diagnosis
CSF
- Minimum volume: >6ml (in adults)
- Storage and transport: Transport to the laboratory urgently. Microscopy to determined cell counts in CSF should be undertaken within 2 hours of collection to ensure the accuracy of the result
Notes
Larger volumes are required to increase diagnostic yield
Blood in Lithium heparin Green topped tubes for mycobacterial blood culture
- Minimum volume: ≥6ml
- Storage and transport: Do not refrigerate
Notes
Use for the diagnosis of disseminated haematogenous tuberculosis (‘miliary TB’)
Pus
- Minimum volume: Largest practical sample
- Storage and transport: Refrigerate if transport to the laboratory will be delayed by more than 1 hour
Notes
Send pus in a sterile container rather than a pus swab
Skin/Tissue
- Minimum volume: Largest practical sample
- Storage and transport: Refrigerate if transport to the laboratory will be delayed by more than 1 hour
Notes
- Collect aseptically
- A caseous portion should be selected if possible: the majority of organisms will be found in the periphery of a caseous lesion
- If sample size is small, add sterile distilled water to prevent desiccation
Bone
- Minimum volume: Largest practical sample
- Storage and transport: Refrigerate if transport to the laboratory will be delayed by more than 1 hour
Notes
Collect aseptically
Urine
- Minimum volume: >5ml
- Storage and transport: Refrigerate if transport to the laboratory will be delayed by more than 1 hour
Notes
- Use white/yellow top sterile containers
- Do not send in red top boric acid containers
- Send 3 samples on consecutive days
- Whole early morning urines are recommended
Bone marrow in Lithium heparin Green topped tubes
- Minimum volume: Largest practical sample
- Storage and transport: Do not refrigerate
Gastric washing (can be used for children where there are problems obtaining sputum)
- Minimum volume: >5ml
- Storage and transport: Transport to the laboratory urgently. Processing of these samples needs to be undertaken within 4 hours of collection to avoid acidic deterioration of organisms
Notes
- Induced sputum is considered preferable to gastric washings, if possible
- Send 3 samples on consecutive days, collected before breakfast
Minimum volume
See above for individual sample types
Limitations and restrictions
Faecal samples are no longer recommended for the investigation of Mycobacteria
Turnaround time
- AFB Smear result: 1 working day
- Mycobacterium tuberculosis complex PCR: 1 working day
- Culture: 2 – 12 weeks
Analysing laboratory
The Regional Centre for Mycobacteria, Freeman Hospital, Freeman Road, High Heaton, Newcastle Upon Tyne, Tyne and Wear, NE7 7DN