TB Testing

TB – it’s a disease you might think has been eradicated if you live in North America … until you apply to volunteer at a hospital.

Mycobacterium tuberculosis is a bacterium that can cause a contagious disease called tuberculosis (TB), where transmission occurs through breathing, coughing, and sneezing of infected people. A challenge with this bacterium is its ability to hide-out in the lungs and wait for an opportune moment to cause disease in its host. This latent form of TB is not transmittable and does not cause immediate disease in the host. The disease progression means there is a great importance for accurate and informative testing processes for latent and active TB.

There are three key reasons a person may be tested for TB – firstly, if there is reason to suspect they may have TB disease; secondly, if a person is immigrating to another country; or thirdly, those working in the health care field. In the case of immigration, the country accepting an applicant may have reason to suspect TB infection based on geography, thus, the testing process could be very similar to the first scenario. Thus, there are some strengths in the current testing process but opportunities for improvements shown through two case studies.

Case 1: Patient suspected that they may have TB disease

To begin, understanding a person’s immunization history is key. In areas of the world with high TB incidence, the BCG vaccine is administered to infants, hours after they are born (See map). This type of medical background information guides Health Canada recommendations. Health Canada recommends the interferon gamma release assay (IGAR) test for those who have been vaccinated twice or have had the Bacille Calmette-Guérin (BCG) vaccine administered after the age of 1 years old. Otherwise, a one-step or two-step tuberculin skin test (TST) is conducted.

Screenshot 2016-07-31 20.29.28

The World Bank incidence data presented on a global map.

The primary difference between the IGAR and TST test is the specificity for the antibodies measured. The TST was created in the early 1900s and are still used today. There may be false positives for those with prior immunization, but there may not. Thus, the IGAR was created in order to provide a more specific test. The IGAR does not measure any of the same strains of Mycobacterium tuberculosis as the BCG vaccine contains.

Case 2: TB test as a requirement for employment or volunteering opportunities

To volunteer in some hospitals, you may require a two-step TB skin test (TST). The requirement for the second test is in order to avoid false negatives. Given the options available the TST is the gold standard for health care providers. It is used to provide a baseline prior to starting one’s occupation. Then it can be used for retesting at regular intervals or in the case of potential contact with an infected patient.

When a TB test is positive

Thus far in the story, TB tests are useful for identifying people with latent TB, but are not effective means of identifying those with active TB. Next steps are required to determine if the individual has a false positive, latent TB, or active TB. Those working in the medical field require a chest X-ray in order to demonstrate the absence of active TB. Those who with positive TB tests and some combination of abnormal chest X-rays or TB infection symptoms will have culture or acid-fast bacteria smear tests conducted. These final tests are the only method for truly identifying those with active TB disease.

Assessment of TB testing

Some success has come from the ease and repetitive ability of TST, financial success can be seen in the innovation of a more specific blood test (IGAR). However, there is room for improvement in the testing process. There are still uncertainties of false negative and positive test results in both TST and IGAR. Moreover, there are many screening steps prior to accurate diagnosis for those who may require treatment. Some steps require more expertise or improved facilities that are not consistently available globally.

For more information:

The Public Health Agency of Canada

Centers for Disease Control and Prevention and their publication

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