Tuberculosis and TB Testing

Written by Daria Sysoeva and Edited by Alexander Alva

Tuberculosis (TB), an infection caused by the bacteria Mycobacterium tuberculosis, has been around for approximately three million years [1] [3]. However, even today, tuberculosis remains a significant public health concern [2]. Most commonly, tuberculosis affects the lungs, but other body systems such as the nervous system and the renal system may also be affected [4].  Tuberculosis can also leave patients with life-long ailments. In some cases, patients who survived tuberculosis develop chronic lung conditions [5]. Diagnostic testing is crucial for the prevention and early treatment of tuberculosis. 

Tuberculosis is spread through air, particularly through respiratory droplets of people infected with TB [3]. Once the inhaled tuberculosis bacteria reaches the lungs, macrophages, or immune cells that kill foreign microorganisms, may engulf the TB bacteria and destroy it. However, the TB bacteria may sometimes express a certain type of protein which allows them to escape the macrophage and spread throughout the body [3] [4]. The immune system’s response in the couple of weeks after the initial infection will determine what kind of TB condition is present: latent TB infection or active TB disease. Latent TB infection occurs if the immune system is able to control the TB infection by restraining the bacteria from spreading throughout the body. In this type of infection, the TB is able to “hide” in a cluster of immune cells called a granuloma and not cause any harm [3]. On the other hand, active TB disease occurs if the immune system is not able to stop the bacteria from replicating and spreading. Latent TB can turn into active TB disease at any time [3].

There are currently two diagnostic tests for TB: the skin test and the blood test. The skin test is formally called the Mantoux tuberculin test and consists of injecting a liquid called tuberculin into the patient’s lower arm. After 48-72 hours, a healthcare professional checks if a reaction has occurred. A round bump present at the site of infection indicates a positive test [3] [6]. The blood test is formally called the Interferon-Gamma Release Assay. This test measures how strongly the immune system reacts to the presence of tuberculosis antigens, proteins that promote an immune system response. A positive test indicates that TB bacteria is present in the patient [3] [7] An important limitation of both tests is that neither test is able to confirm whether or not the patient has latent TB or active TB disease [3]. After a positive test, the patient requires a chest X-ray or sputum test to confirm if their TB is latent or active [10]. Like all diagnostic tests, the diagnostic tests for tuberculosis are not perfect. People who have received the Bacillus Calmette–Guérin (BCG) vaccine, a vaccine against TB that is administered in countries where TB is most common, may test positive on skin tests without being infected with TB [8]. However, TB skin tests are more cost-effective than blood tests; for this reason, skin tests are more commonly used in less-developed countries with high rates of tuberculosis [9]. Ultimately, further research is needed into the development of more accurate and affordable diagnostic testing for tuberculosis to improve TB diagnosis, care, and prevention.

References:

1. “World TB Day.” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 2016, www.cdc.gov/tb/worldtbday/history.htm#:~:text=Johann%20Schonlein%20coined%20the%20term,%E2%80%9Cschachepheth%E2%80%9D%20in%20ancient%20Hebrew. Accessed 16 May, 2021. 
2. “Tuberculosis.” World Health Organization, World Health Organization, 2020, www.who.int/news-room/fact-sheets/detail/tuberculosis#:~:text=Worldwide%2C%20TB%20is%20one%20of,all%20countries%20and%20age%20groups. Accessed 16 May, 2021. 
3. Agyeman, A. A, Ofori-Asenso, R. (2017). Tuberculosis-An Overview. Journal of Public Health and Emergency, 1:1. 
4. Smith, I. (2003). Mycobacterium tuberculosis Pathogenesis and Molecular Determinants of Virulence, Clinical Microbiology Reviews, 16:463-496. 
5. Loddenkemper, R., Lipman, M., Zumla, A. (2016). Clinical Aspects of Adult Tuberculosis, Cold Spring Harbor Perspectives in Medicine, 6:a017848.
6. “Testing for TB Infection.” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 2021, www.cdc.gov/tb/topic/testing/tbtesttypes.htm. Accessed 16 May 2021. 
7. “Tuberculosis”. BioReference Laboratories, www.bioreference.com/physicians/diseases-testing/tuberculosis/. Accessed 16 May 2021. 
8. Cohn, D. L. (2001). The Effect of BCG Vaccination on Tuberculin Skin Testing Does It Matter? American Journal of Respiratory Care and Critical Care Medicine, 164:915-91610. 
9. Gualano, G., Mencarini, P., Lauria, F. N., Palmieri, F., Mfinanga, S., Mwaba, P., Chakaya, J., Zumla, A., Ippolito, G. (2019). Tuberculin skin test – Outdated or still useful for Latent TB infection screening? International Journal of Infectious Diseases, 80:S20-S22.
10. “Tuberculosis”. Mayo Clinic, 2021 www.mayoclinic.org/diseases-conditions/tuberculosis/diagnosis-treatment/drc-20351256. Accessed 16 May, 2021.