How do we Diagnose TB?
Active TB Diagnosis
Testing for active tuberculosis (TB) is indicated in everyone with signs and symptoms of TB or considered to be at high risk of TB disease. In Canada, the standard testing algorithm for active TB includes the following tests:
- Chest radiography
- Sputum smear microscopy
- Mycobacterial culture and phenotypic drug susceptibility tests (DST)
- Nucleic acid amplification tests (NAATS)
Latent Tuberculosis Infection (LTBI) Diagnosis
Main goal of testing for latent TB infection (LTBI) is to identify individuals who are at increased risk for progressing to active TB disease. LTBI can become active at any time with the risk of activation being highest in the first two years following initial infection. About 10% of people with LTBI will develop TB disease at some point in their lives. This risk is increased in people with HIV/AIDS or other chronic conditions that weaken the immune system.
In Canada, two accepted tests for the identification of LTBI are:
- Tuberculin skin test (TST)
- Interferon gamma release assay (IGRA)
Click for Resources
Learn more at:
Canadian Communicable Disease Report: Latent TB Infection Overview
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5764738/
The WHO End TB Strategy
https://www.who.int/tb/strategy/end-tb/en/
The Canadian Federal framework for Action on TB Prevention and Control
https://www.canada.ca/en/public-health/services/infectious-diseases/tuberculosis-prevention-control-canada.html
LTBI diagnosis: https://www.tbinfocus.ca/tb-diagnosis/
LTBI treatment: https://www.tbinfocus.ca/tb-treatment/
Chest Radiography
Recommendations from the Canadian TB Standards:
Chest radiography (posterior-anterior and lateral views) is the usual first step in evaluation of an individual with pulmonary symptom. Limitations include: poor specifity (range between 60-70%), moderate sensitivity (70-80%), and variable interpretations and poor agreement of x-ray readings.
Sputum Smear Microscopy
Sputum smear microscopy is the most widely used test for TB disease. Smear microscopy is rapid, inexpensive and identifies the most infectious TB patients.
Limitations: modest sensitivity (20%-80% depending upon the type of specimen, patient population, stain used and the experience of the microscopist); cannot be used to determine drug resistance
Recommendations from the Canadian TB Standards: Everyone with suspected TB should undergo testing with at least three concentrated fluorescent smears. Spontaneous or induced sputum specimens can be used. Smear microscopy is performed routinely on all specimens submitted to the mycobacteria laboratory for testing.
Nucleic Acid Amplification (XPERT MTB/RIF)
The amplification of nucleic acids for the diagnosis of TB or to detect drug resistance is a sensitive method and produces a much faster result than conventional culture methods. In general, NAATs require sophisticated laboratory infrastructure and highly skilled technicians. Currently, Xpert MTB/RIF is the only fully automated, cartridge-based NAAT on the market. Unlike conventional NAATs, the Xpert MTB/RIF is not dependent on reference laboratories or a high degree of technical expertise.
Recommendations from the Canadian TB Standards:
At least one respiratory sample should be tested with a NAAT in all new, smear-positive cases. NAA testing may be performed in smear-negative patients upon request by the physician or the TB control program.
NAAT results are not recommended for monitoring TB treatment response.
In settings where there is currently no on-site capacity for routine smear microscopy and culture (for example, hospitals in the North serving Aboriginal populations), an automated, cartridge-based NAA test can be used to make rapid decisions on TB treatment and isolation pending routine smear and culture results.
All Xpert MTB/RIF results should be confirmed by routine smears and cultures.
Culture
Mycobacterial culture is the most sensitive and the current gold standard method for the detection of active TB disease. In Canada, every specimen that is sent for smear microscopy is submitted for culture. The use of culture remains necessary for the definitive diagnosis of smear-negative TB.
The benefits of culture include identification, drug susceptibility testing and further use of culture isolates for molecular epidemiology using DNA fingerprinting.
Recommendations from the Canadian TB Standards:
Every specimen that is sent for smear microscopy should be submitted for culture on one solid medium and one liquid medium.
Interferon Gamma Release Assay (IGRA)
IGRAs are in vitro blood tests of cell-mediated immune response; they measure T cell release of interferon-gamma (IFN-gamma) following stimulation by antigens specific to M. tuberculosis. IGRAs are surrogate markers of Mycobacterium tuberculosis infection and indicate a cellular immune response to M. tuberculosis. IGRAs cannot separate LTBI from active TB disease and therefore have no value for active TB detection.
Tuberculin Skin Test (TST)
The tuberculin skin test (TST), also called the Mantoux tuberculin skin test, is performed by injecting a small amount of fluid (called tuberculin) into the skin on the lower part of the arm. A person given the tuberculin skin test must return within 48 to 72 hours to have a trained health care worker look for a reaction on the arm.
When interpreting a positive TST, it is important to consider the size of induration, positive predictive value and risk of disease if the person is truly infected.
Positive skin test: This means the person’s body was infected with TB bacteria. Additional tests are needed to determine if the person has latent TB infection or TB disease. Negative skin test: This means the person’s body did not react to the test, and that latent TB infection or TB disease is not likely.
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