Laboratory: Clinical Microbiology
Test Name:
Test Code: AFBSK
Clinical Information:
Test Indications:
Skin biopsy samples may be submitted from patients suspected of having a cutaneous infection caused by a nontuberculous mycobacteria (NTM), such as M. fortuitum, M. abscessus, M. chelonae, M. marinum and M. ulcerans, or tuberculosis (TB) affecting the skin, specifically cutaneous TB.

Culture from the drainage material or a tissue biopsy are recommended for the identification of NTM. (American Thoracic Society Documents)
The Canadian Tuberculosis Standards recommend that:
Cutaneous tuberculosis is rare and the diagnosis depends on both the histopathology and mycobacterial smear/culture results from a biopsy sample.
"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 skin sample culture is evaluated by microscopy and culture.  Samples are decontaminated and concentrated prior to 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.
The nucleic acid amplification test (NAAT)/PCR is NOT validated for skin samples.  The in-house NAAT only identifies the presence of M. tuberculosis.  Following consultation with the microbiologist-on-call, any requests for NAAT for NTM are referred to the National Reference Centre for Mycobacteriology - National Microbiology Laboratory (NRCM-NML).
The identification of M. tuberculosis, Mycobacterium avium complex and Mycobacterium gordonae is performed in-house.  Other AFB-positive organisms are referred to the 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.
Collection Devices:
Swabs are SUBOPTIMAL specimens.  Dry swabs and ear swabs will be rejected as inappropriate specimens.
Specimen Required: Skin biopsies - as much as possible (Adult and Pediatric)
Tissue sample may be submitted fresh or in a small amount of normal saline.  Fluid should be aspirated by syringe following the removal of the surface exudate with sterile saline or 70% alcohol. 
Do not add preservatives or additives. Formalin-fixed samples cannot be cultured and will be rejected. 
Reference Values:
Microscopy:  No acid-fast bacilli observed.
Culture:  No acid fast bacilli isolated after 7 weeks of incubation.  
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.
See Also:
More Information:
Specimen Handling:
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.