MYCOBACTERIA (ACID-FAST BACILLI [AFB]), CULTURE, TISSUE
Test Code: AFBT
AFB tissue samples may be submitted from the affected body site for patients suspected of non-respiratory tuberculosis. AFB tissue samples may also be submitted from the affected body site for patients suspected of non-respiratory nontuberculous mycobacterial (NTM) disease, such as NTM lymphadenitis.
The Canadian Tuberculosis Standards recommend that:
"Whenever practical, every effort should be made to obtain clinical samples for both mycobacteriologic (AFB smear and culture) and histopathologic tests. Drug susceptibility testing can only proceed with a viable culture, the results of which can have important treatment implications." (Strong recommendation, based on strong evidence)
"Every presumed case of nonrespiratory TB should be assessed for pulmonary TB. How infectious the possible case is depends upon respiratory involvement. Pulmonary involvement in patients with nonrespiratory TB disease can range from 10% to 50%, thus it may be possible to secure a diagnosis of TB with sputum assessment and avoid the need for more invasive sampling." (Strong recommendation, based on strong evidence)
"In general, tissue biopsy yields positive culture results more often than fluid aspiration; both are superior to swabs".
Shared Health Testing Process - Every AFB tissue culture is evaluated by microscopy and culture. Smears are reported semi-quantitatively (Negative or 1+ to 4+). Cultures are incubated for 8 weeks and reported as soon as growth is detected. According to the Canadian Tuberculosis Standards "Mycobacterial culture is the most sensitive and the current gold standard method for the detection of active TB disease" and the role of "nucleic acid amplification (NAA) in nonrespiratory specimens remains incompletely defined. Its major advantage is a rapid diagnosis…". All microscopy smear positive AFB samples receive NAA/PCR testing. The in-house NAAT detects M. tuberculosis (not all Mycobacteria). AFB smear negative samples require a microbiologist consult if a NAAT is required. The sensitivity of smear negative samples is reduced (75% compared to up to 100% for smear positive samples).
The identification of M. tuberculosis, Mycobacterium avium complex and Mycobacterium gordonae is performed in-house. Other AFB-positive organisms are referred to the National Reference Centre for Mycobacteriology - National Microbiology Laboratory (NRCM-NML) for identification. Susceptibility testing for M. tuberculosis is performed routinely by the NRCM-NML. Susceptibility testing for nontuberculosis mycobacteria may be requested by consultation with the Shared Health Microbiologist-on-call.
If Mycobacterium ulcerans is suspected clinically, the laboratory must be notified as prolonged culture incubations may be required.
Patient Preparation Instructions:
Patients' sample collection should be conducted in accordance with Infection and Control Practices. The WRHA Acute Infection Prevention and Control Manual states that "when collecting specimens for suspected or active TB, specimens must be collected utilizing Airborne Precautions regardless of age".
Accurate patient identification must be made prior to sample collection. Patient identification should be done in accordance with site policy.
Samples and requisitions must be labeled/completed in accordance with the Shared Health Specimen Acceptance Policy.
Swabs are SUBOPTIMAL specimens. Dry swabs and ear swabs will be rejected as inappropriate specimens.
Tissue biopsy samples - As much as possible (1 gram, if possible) (Adult and Pediatric)
Sample may be submitted fresh or in a small amount of normal saline. Do not add preservatives or additives. Formalin-fixed samples cannot be cultured and will be rejected.
Microscopy: No acid-fast bacilli observed.
Culture: No acid fast bacilli isolated after 7 weeks of incubation.
NAAT: Negative for Mycobacterium tuberculosis complex DNA by Real-time PCR.
Weekdays (Monday - Friday). Mycobacteriology cultures are only performed at the Health Sciences Centre in Winnipeg. AFB positive smears of specimens are phoned to the nursing unit immediately. Culture positive index cases of M. tuberculosis are phoned to the nursing unit immediately. Positive M. tuberculosis NAAT results of index cases are phoned to the nursing unit immediately.
Samples should be refrigerated for storage.
Ensure samples that are being sent to a referral laboratory are packaged in accordance with Transport of Dangerous Goods recommendations for diagnostic samples.