Many schools and workplaces in the United States and abroad give TB skin tests. This is a preventive measure designed to limit the spread of active TB disease. Active TB can be spread through the air and can become a huge public health problem if not immediately identified and treated.
TB skin tests use tuberculin, a non-infectious protein made by the bacteria that cause TB. For the most common TB test, a health care worker will use a tiny needle to inject a small amount of tuberculin just under the surface of the skin, usually on the inside of one of your forearm. You must return 48 to 72 hours after the injection to have your skin test read.
If you are currently infected with TB or ever have been infected with TB, your immune system will mount a swift response against the tuberculin, and a bump will form at the injection site over the next day or two. The bump is from inflammation, one of the ways your body's immune system responds to germs that make you sick.
If you have not been infected with TB, your body's immune system will not recognize the tuberculin as being harmful and therefore will not mount an immune response against it. A health care professional will look at the bump on your skin and determine whether it is large enough to represent a positive result.
A positive skin test does not necessarily mean that you have active disease. It is much more likely that you have a latent TB infection, which means you have been infected but are not sick and cannot pass the disease on to others. A latent TB infection should be treated so it does not develop into active disease. A positive skin test means that you'll have to go through more tests. These additional tests are required to determine whether you have the latent (inactive) or the active form of the disease.
A negative skin test does not mean you definitely are not infected with the bacteria that cause TB. You may have been infected too recently for the test to be able to detect the infection. People with severely weakened immune systems sometimes are unable to mount the inflammatory reaction that causes a positive result, so they can have a negative skin test even if they are infected with the bacteria.
If you have had the BCG vaccination, you may have a positive skin test. If you have had the BCG vaccination but think you may have TB, you will need further testing.