MYCOBACTERIA (ACID-FAST BACILLI [AFB]), CULTURE, GASTRIC ASPIRATE/LAVAGE/WASH
Test Code: AFBR
The Canadian Tuberculosis Standards state that "testing for active tuberculosis (TB) is indicated in everyone with signs and symptoms of TB or considered to be at high risk for TB.
Gastric aspirates for TB diagnosis are typically submitted from young children who are unable to produce sputum.
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. As per the Canadian Tuberculosis standards, microscopy was positive in 0%-21% (median 7%), and culture was positive in 0%-75% (median 20%) of children with a clinical diagnosis of likely TB.
Laboratory Testing Process - The testing process for gastric aspirate/washes is identical to the respiratory specimen procedures except the gastric samples are neutralized with 100 mg sodium carbonate immediately upon receipt. Samples are decontaminated and concentrated prior to 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.
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.
1 - Sterile Specimen Container (100 mL)
Gastric Aspirate/Wash - Pediatric: Maximum volume of specimen is 15 mL (plus 20-50 mL of lavage fluid); Adult: Same as pediatric, if required.
Specimens must be neutralized with sodium carbonate. Specimens that are not neutralized within 4 hours of collection 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.
Specimens should be stored at room temperature.
Ensure samples that are being sent to a referral laboratory are packaged in accordance with Transport of Dangerous Goods recommendations for diagnostic samples.