The tuberculosis skin test is a test that can show if you have been infected with tuberculosis (TB) bacteria. The test is called the Mantoux tuberculin skin test (TST).
Tuberculosis usually causes lung disease, although it can affect almost any part of the body. TB infections are not common in the US. However, an active TB infection can be a very serious illness. The TB skin test can find most TB infections before the infection is serious enough to cause any problems.
Most people who are infected with TB bacteria have a form of infection called latent TB. They usually do not develop active TB, which means that they have no signs or symptoms of illness. Their body’s defenses control the infection by building a wall around the bacteria. This doesn’t get rid of the bacteria, but the bacteria stop growing. If you have latent TB and become weak or ill, or your immune system stops working well, the TB bacteria can become active. The bacteria can then multiply and you may get sick.
If the test finds that you have TB bacteria, you can be treated with medicine that will keep you from having an active infection and getting sick. The medicine will also keep the infection from spreading to others.
You should get tested for TB if you:
Usually no preparation is needed for this test. Just be sure you can come back to have your skin checked in 48 to 72 hours. Otherwise you will need to repeat the test.
Talk to your healthcare provider if you have any questions about the test.
Your healthcare provider will use a needle to inject a small amount of fluid into the top layer of your skin. The fluid contains protein from the tuberculosis bacteria. It is like getting a shot.
Your healthcare provider will want you to come back to the office in 2 to 3 days to check the area that was injected.
A 2-step test method is used for older people because they may have a poorer immune response. If the first test is negative, the test is repeated in 7 to 10 days.
If after 2 or 3 days you have a red, raised, firm area around the test site, then you have probably been infected with TB. The lump will be measured to see if it is large enough to be seen as a positive result. A positive result does not necessarily mean you have an active TB infection. Most people with positive tuberculin tests don’t have active TB. A positive test without active disease simply means that sometime in the past you were exposed and infected with TB, but your body’s immune system is keeping the bacteria under control.
If you are not infected, there may be no reaction in the area of the injection. This is a negative result. You may also have no reaction if you were just recently infected (in the last 6 weeks).
Sometimes it takes longer than 3 days for the skin to show a reaction. Tell your provider if any redness appears after the first 3 days. Your provider will decide if the redness is from a TB infection or some other cause.
Sometimes the test may give a false-positive or false-negative result. This means that the skin may show a reaction even though you are not infected (false positive), or there is no reaction even though you are infected (false negative).
This test does not tell whether the infection is latent or active. Other tests, such as a chest X-ray and a sample of sputum, are needed to see if the infection is an active disease.
Many people born outside of the US have been vaccinated against TB with the Bacillus Calmette-Guérin (BCG) vaccine. This vaccine is not routinely given in the U.S. The BCG vaccine may cause the skin to have a reaction after a TB skin test, so that it looks like you have TB even when you do not. If you have had the BCG vaccine, be sure to tell your healthcare provider.
If the test is positive, close family members should also have the test.
Test results are only one part of a larger picture that takes into account your medical history and current health. Sometimes a test needs to be repeated to check the first result. Talk to your healthcare provider about your result and ask questions such as:
Talk to your healthcare provider if you have any questions about the test.
