MYCOBACTERIA (ACID-FAST BACILLI [AFB]), CULTURE, GASTRIC ASPIRATE/LAVAGE/WASH

Test Code: AFBR

Test Indications:
The Canadian Tuberculosis Standards state that "testing for tuberculosis (TB) is indicated in everyone considered to be at high risk for TB disease or with signs and symptoms of TB.
Gastric aspirates for TB diagnosis are typically submitted from young children who are unable to produce an expectorated sputum.
 
Recommendations:
Samples should be obtained prior to the commencement of therapy.
Gastric aspirates/washes may be submitted instead of sputum from children or adult patients who cannot produce a sputum (expectorated or induced).
Gastric aspirates/washes must be collected early in the morning prior to eating.
Samples must be neutralized as soon as possible (within 4 hours by the laboratory) to prevent the gastric acid from killing M. tuberculosis.  NOTE: the Mycobacteriology laboratory hours begin at 7:45 am Monday-Friday.   
 3 gastric aspirates/washes should be collected and submitted for testing by microscopy and culture.
Current practice by the Pediatric Respirology Department at the Children's Hospital is to collect gastric aspirates following recommendations similar to the initial diagnostic sputum samples (i.e., 3 gastric aspirates/washes, at least 1 hour apart, first thing in the morning and prior to eating).  For more complex cases (as determined by the Respirology service or the Infectious Diseases service) of an admitted patient, additional samples are also to be collected on day 2 and 3.
The sensitivity of gastric aspirates and lavages is reported to be lower than sputum samples. The Canadian Tuberculosis standards reference a study that “found that samples obtained from gastric aspiration or sputum induction had low positivity for both GeneXpert and mycobacterial culture”.
Laboratory Testing Process - Gastric samples are neutralized with 100 mg sodium carbonate immediately upon receipt. Every sample is evaluated by microscopy and culture. Smears are reported semi-quantitatively (Negative or 1+ to 4+). Cultures are incubated for 7 weeks and reported as soon as growth is detected.
All microscopy smear positive AFB samples receive nucleic acid amplification testing (NAAT). AFB smear negative samples require a microbiologist consult if a NAAT is required. The in-house NAAT detects M. tuberculosis and Mycobacterium sp. DNA.
M. tuberculosis are routinely referred to the National Reference Centre for Mycobacteriology-National Microbiology Laboratory (NRCM-NML) for susceptibility testing. Susceptibility testing for non-tuberculous mycobacteria may be requested by consultation with the Shared Health Microbiologist-on-call.  

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.
Refer to the Operational Directive for Gastric Aspirates for specimen collection guidance. 
Gastric aspirates must be collected in the early morning, prior to eating or drinking anything overnight.  The ideal timing is at the time of waking.  This prevents the stomach from emptying.
Aspirate the stomach contents first.  Then, instill 20-50 mL of sterile distilled water and aspirate back.  Add this aspirate to the first specimen in the sterile specimen container.   

Microscopy: No acid-fast bacilli observed.
Culture: No acid fast bacilli isolated after 7 weeks of incubation.
NAAT: Negative for Mycobacterium tuberculosis complex DNA and Negative for Mycobacteria spp. DNA.

Weekdays

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.

Specimen Handling:
Specimens should be stored at room temperature.