Above: Administering a PPD test for tuberculosis (Wikimedia Commons/US Centers for Disease Control and Prevention)

Did you know? One-third of the world’s population is infected with tuberculosis.If you’ve ever volunteered or worked in a hospital or a homeless shelter, or if you’ve traveled to certain parts of the world, chances are you’ve had a tuberculosis (TB) test. But why would you need to get tested for a disease rarely seen in North America? And how does the test work?

The test: How it works

A TB test is also sometimes called a PPD test since it contains the protein purified derivative (PPD) from Mycobacterium tuberculosis, the bacterium that causes TB. The test involves injecting a liquid containing PPD into the top layer of your skin, where a bubble forms. The bubble will go down after a few hours, and you may also experience some redness.

If you don’t have TB, nothing else will usually happen, because your body will not react to the PPD. However, if you do have TB, the area around the injection site will become raised and hard within two or three days. This means that your body reacted to the PPD.

Did you know? Worldwide, tuberculosis is the second greatest cause of death from an infectious agent, after HIV/AIDS. Tuberculosis is the leading cause of death from a bacterial infection.Whether the test is actually considered positive depends on the size of the raised area. Usually, a diameter of 15mm or more indicates a TB infection. If you do get a positive test, then you may have to undergo more tests such as blood tests or chest X-rays to confirm a positive diagnosis. False positives may occur in those who have had a nontuberculous mycobacterium infection or who have received a TB vaccine.

So what exactly goes on under your skin during a TB test? When you are exposed to Mycobacterium tuberculosis, your body recognizes a part of the bacterium called an antigen. This provokes a reaction from body’s immune system, which produces lymphocytes (a type of cell) that react specifically to the Mycobacterium tuberculosis antigen. These cells remain in your system for years and when they come into contact with the PPD used in the TB test, they react. This causes the raised, hard area to form at the site of the injection, indicating that you may be infected.

Did you know? Worldwide, tuberculosis is the leading cause of death for people suffering from HIV/AIDS.Why you need to take it: Latent tuberculosis

In 1907, the term “latent tuberculosis” was first introduced to describe a person who doesn’t show any signs of tuberculosis disease, but who does test positive for TB. Fortunately, tuberculosis remains latent in 90% of those infected with Mycobacterium tuberculosis.

In latent TB, the bacterium lies dormant, the disease cannot be passed to others, and there are no signs or symptoms. However, the disease can become active in those with latent TB, especially if they are malnourished or they suffer from HIV/AIDS. Once it becomes active, the disease can easily be passed to others simply by coughing.

Since TB can lie dormant for years, it’s important to check for TB before working with people with weakened immune systems. If you happened to have latent TB and it became active, you could pass it to others who would have a much harder time fighting off the disease. This is why a PPD test is required before working in locations, including hospitals and homeless shelters, where you are likely to be in contact with people especially vulnerable to TB.

Did you know? A PPD test for tuberculosis is also called a Mantoux test. Charles Mantoux was a French medical doctor who helped develop the test in the early 1900s.In 2012 TB caused 1.3 million deaths worldwide, predominately in Asia and Africa. In fact, just 22 countries account up 80% of the world’s TB cases. You should discuss taking a TB test with your health care provider before travelling to these regions.

Though often deadly, TB can be treated. This involves taking antibiotics, which often have bad side effects, for at least 6 months. However, some bacteria are becoming resistant to existing antibiotics, which makes treating TB more and more difficult.

Fortunately, active TB is quite rare in Canada. Only about 1600 cases are reported each year. Nevertheless, it’s very important to continue testing for latent TB to ensure the disease remains a rarity here!


General information

Coughing up the facts on XDR-TB (CurioCity by Let’s Talk Science) Health Concerns: Tuberculosis (Health Canada) TB and HIV Coinfection (US Centers for Disease Control and Prevention) Trade, foreign policy, diplomacy and health: Tuberculosis control (World Health Organization) Treatment of Latent Tuberculosis Infection: Maximizing Adherence (US Centers for Disease Control and Prevention) Tuberculosis (TB) (World Health Organization) Tuberculosis Fact Sheet (World Health Organization) Tuberculosis Fact Sheets (Public Health Agency of Canada)

Scholarly publications

American Thoracic Society. 2000. Diagnostic Standards and Classification of Tuberculosis in Adults and Children. American Journal of Respiratory and Critical Care Medicine. 161:1376-1395. Daniel TM. 2006. The history of tuberculosis. Respiratory Medicine. 11:1862–70. Herzog BH. 1998. History of tuberculosis. Respiration. 1:5-15.

Kelly Resmer

Kelly is a chemistry undergraduate laboratory instructor in Halifax.  She loves working with students in the lab, watching chemistry happen! She has a PhD in chemistry and is very interested in studying and learning about bacteria, the good and the bad ones! 

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