What is it?

The tuberculosis (TB) skin test can tell if you have TB bacteria in your body. Tuberculosis (TB) is a bacterial infection that can damage the lungs or other parts of the body.

Many people are exposed to TB and test positive for TB bacteria in their bodies, but they don't get the disease. TB bacteria can stay in your body without making you sick. This is because your immune system can keep TB in check.

Why is the test done?

A doctor may want you to have this test if you have been in close contact with someone who has tuberculosis (TB). You may also have the test if you have symptoms that might be causing TB. These include a cough that doesn't go away and unexplained weight loss.

How do you prepare for the test?

In general, there's nothing you have to do before this test, unless your healthcare provider asks you to.

How is the test done?

For a tuberculin skin test, you sit down and turn the inner side of your forearm up. The skin where the test is done is cleaned and allowed to dry. A small shot of the tuberculosis antigen (purified protein derivative, or PPD) is put under the top layer of skin. The fluid makes a little bump (wheal) under the skin. A circle may be drawn around the test area with a pen.

What happens after the test?

• Do not cover the site with a bandage.
• You must go back to the clinic 48 to 72 hours (2 to 3 days) after the test to have a nurse look at the test. If you have TB in your body, a firm red bump will form at the shot site within 2 days.
• If the test shows that you are infected with TB (positive), your doctor probably will order more tests. A TB-positive skin test can't tell when you became infected with TB. And it can't tell whether the infection can be passed to others.

How can you care for yourself at home?

• Do not scratch the test site. Scratching it may cause redness or swelling. This could affect the test results.
• To ease itching, put a cold face cloth on the site. Then pat the site dry.
• Do not cover the test site with a bandage or other dressing.