Testing for Tuberculosis: Skin Test

TB skin test

The TB skin test is also known as the tuberculin skin test. If you receive a TB skin test, you will have two visits with your health care provider.

First visit

During the first visit, a health care provider uses a small needle to put some testing material under the skin on the lower part of your inner arm. The testing fluid is called tuberculin.

Second visit

After two or three days, you must return to the clinic to have the skin test read. A trained health care provider will measure the size of the bump or reaction. A positive or negative test result depends on the size of the bump or reaction.

If you cannot return after two or three days to have the test read, you will need to have a skin test again.

Who can get a TB skin test

Most people can get a TB skin test. This includes:

• Infants,
• Children,
• Pregnant people,
• People with HIV infection, and
• People who had a negative TB skin test or an undocumented TB skin test.

You should not get a TB skin test if you have had a severe reaction, such as a severe allergic reaction, in the past. Talk to your health care provider about the TB blood test.

If you received the TB vaccine

Bacille Calmette-Guérin (BCG) is a vaccine for TB disease. The vaccine is not generally used in the United States. It is given to infants and small children in countries where TB is common. It protects children from getting severe forms of active TB disease, such as TB meningitis.

Tell your health care provider if you have received the TB vaccine, especially if you are getting tested for TB infection because it can cause a false positive TB skin test reaction. TB blood tests are the preferred tests for people who have received the BCG TB vaccine.

How to get tested

You can get tested for TB at the health department or at your health care provider's office. Your health care provider will choose the TB test that is best for you. Public and private health care plans may cover TB testing costs.

Two-step TB skin test

Your health care provider may perform a two-step TB skin test if you are going to be screened for TB periodically (for example, if you are a health care worker).

Some people with inactive TB, also called latent TB infection, have a negative reaction to the TB skin test when tested years after being infected. However, if they are tested again within a year of the first test, they may have a positive reaction. The first TB skin test can "trigger the memory" of the immune system, boosting its ability to react to the second TB skin test.

It may appear that these people were infected between the first and second tests. However, the second positive test reaction is actually a boosted reaction due to TB infection that occurred a long time ago.

The two-step TB skin test can lower the chance that a boosted reaction from an old TB infection will be misinterpreted as a recent infection. If the reaction to the first-step TB skin test is classified as negative, a second-step TB skin test is given one to three weeks after the first test is read.

Understanding TB skin test results

Positive TB skin test

A positive skin test result for TB infection means you have TB germs in your body.

Negative TB skin test

A negative skin test result for TB infection means inactive TB or active TB disease is unlikely, but your health care provider may do more tests, especially if:

• You have symptoms of active TB disease, like
  • Coughing,
  • Chest pain,
  • Fever,
  • Weight loss, or tiredness.
• You have HIV.
• You were recently exposed to TB germs.

After you get your results

If you have a positive TB skin test result, you have TB germs in your body. Your health care provider will do other tests to determine if you have inactive TB or active TB disease. These tests may include a chest x-ray, and a test of the sputum (phlegm) you cough up.

Your health care provider will discuss your diagnosis with you and recommend treatment.

Resources