A tuberculin skin test is done to see if you have ever had tuberculosis (TB). The test is done by putting a small amount of TB protein (antigens) under the top layer of skin on your inner forearm. If you have ever been exposed to the TB bacteria (Mycobacterium tuberculosis), your skin will react to the antigens by developing a firm red bump at the site within 2 days.

The TB antigens used in a tuberculin skin test are called purified protein derivative (PPD). A measured amount of PPD in a shot is put under the top layer of skin on your forearm. This is a good test for finding a TB infection. It is often used when symptoms, screening, or testing, such as a chest X-ray, show that a person may have TB.

A tuberculin skin test cannot tell how long you have been infected with TB. It also cannot tell if the infection is latent (inactive) or is active and can be passed to others.

Why It Is Done

A tuberculin skin test is done to find people who have tuberculosis (TB), including:

• People who have been in close contact with someone known to have TB.
• Health care workers who are likely to be exposed to TB.
• People with TB symptoms, such as an ongoing cough, night sweats, and unexplained weight loss.
• People who have had an abnormal chest X-ray.
• People who have had a recent organ transplant or have an impaired immune system, such as those with human immunodeficiency virus (HIV).

A tuberculin skin test should not be done for people:

• With a known TB infection.
• Who have had a previous severe reaction to the TB antigens.
• Who have a skin rash that would make it hard to read the skin test.

How To Prepare

Before having a tuberculin skin test, tell your doctor if you:

• Have symptoms of tuberculosis (TB), such as an ongoing cough, night sweats, or unexplained weight loss.
• Have had a severe reaction to a tuberculin skin test in the past.
• Have had TB in the past.
• Have risk factors for TB, which are things that increase your risk. Risk factors for TB include:
  • Contact with a person who has TB.
  • A job as a health care worker that may cause you to be exposed to people with TB.
  • Having lived or traveled in a country where TB is common.
• Have been given a TB vaccination. The vaccine contains a bacteria called BCG (bacille Calmette-Gu�rin) that is closely related to the bacteria that cause TB.
• Have been treated with medicines, such as corticosteroids, that can affect your immune system.
• Are infected with the human immunodeficiency virus (HIV).
• Have a skin rash that may make it hard to read the skin test.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).