TB skin test

A TB skin test looks for TB infection. TB proteins are injected under the surface of the skin. If a person is infected with TB, a hard swelling (induration) will develop at the site of the injection in 48 to 72 hours. A health care professional must measure the size of the reaction and tell you if the test is positive.

A positive test

A positive TB skin test usually means that you have TB infection. More tests should be done to make sure you don't have TB disease. Your doctor may order a chest x-ray or a test of your sputum (phlegm) to look for TB bacteria. A positive test without TB infection can happen in people who have been vaccinated with BCG or who have been infected with other TB-like mycobacteria.

A negative test

A negative TB skin test usually means a person is not infected with TB. A negative test can happen in a person who has been recently infected. It usually takes 2 to 12 weeks after exposure to a person with infectious TB disease for the skin test to become positive.

A negative test can also happen in an infected person who has a weakened immune system.

If you have TB and HIV infection or TB disease, your skin test may be negative even if the TB germ is in your body.

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