What Is a TB Test?


Tuberculosis, or TB, is a very serious condition caused by a bacterial infection. It primarily attacks the lungs, but it also attacks the nervous system. If left untreated, it can lead to death. Its symptoms include chronic cough with blood-tinged sputum, fever, night sweats, and weight loss. Luckily, there is a test that can tell you whether or not you’ve been exposed to TB.

What is Tuberculosis?

Tuberculosis is a serious condition. Family Doctor defines tuberculosis as “an infection caused by a bacteria (germ). Tuberculosis usually affects the lungs, but it can spread to the kidneys, bones, spine, brain and other parts of the body.” Its symptoms include pain, difficulty in breathing, and kidney failure. They also state that there are two types of TB, a TB infection and active TB. A TB infection simply means that the bacteria are present in your body. You will show no signs or symptoms and you will not be contagious. Active TB is the TB that will make you sick and is contagious.

Why Would I Need A Test?

TB tests are taken for many reasons. One reason to take a TB test is if you believe you have been exposed to TB. Those who are routinely tested include those in the medical fields such as doctors or nurses. People who are going to work in a group home setting, such as a mental health home, sanitarium, or nursing home will often be tested for TB so they do not bring any infectious diseases into the home. Some people will get themselves tested if they are suffering from signs or symptoms of TB which including coughing and pain.

What is The Test?

The TB test is remarkably simple. The American Lung Association defines that test as “(an injection of) a small amount of tuberculin under the superficial layers of the skin. The test is then read by a trained individual 48 to 72 hours later.” There are two possible results for a TB test. A negative result is when the area of the skin affected shows no signs of bruising or scaring. A positive test ends up with a bump on the skin. There may also be bruising, scarring, and the bump may be painful to touch. The test reader then decides how serious the result is by the size of the bump.

What Does a Positive Test Mean?

A positive test can mean many things. Medline Plus states that “The results of the test depend on the size of the skin reaction and on the person being tested.” A reaction of 5 mm of swelling is generally considered not serious and safe. However, this reaction will be considered positive if the person has HIV, has been taking steroids, or if they have been in contact with someone who has active TB. A reaction of great than or equal to 10 mm is considered positive in people with diabetes or kidney failure or health care workers. A reaction of 15 mm or more swelling is positive in people with no known risk of TB. A positive result doesn’t automatically mean that you have active TB. An x-ray will be taken of your chest to see if you have active TB.


The risks for the test are minimal. There is no risk of obtaining TB from the test. There is a small risk of severe redness and swelling of the arm that goes beyond the testing area. This is more common in people who had had a previous positive test and are then tested again. Sometimes this reaction happens in people who have not been tested, but this is very rare.

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