Tuberculin Skin Test

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

In general, there's nothing you have to do before this test, unless your doctor tells you to.

How It Is 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.

How It Feels

You may feel a quick sting or pinch from the needle.

Risks

There is a very slight risk of having a severe reaction to the tuberculin skin test, especially if you've had tuberculosis (TB). An allergic reaction can cause a lot of swelling and pain at the site. You may have a sore.

You cannot get a TB infection from the tuberculin skin test. That's because no live bacteria are used for the test.

A tuberculin skin test is safe during pregnancy or breastfeeding.

Results

Redness alone at the skin test site usually means you haven't been infected with TB bacteria. A firm red bump may mean you have been infected with TB bacteria at some time. The size of the firm bump (not the red area) is measured 2 to 3 days after the test to find out the result. Your doctor will consider your chance of having TB when looking at the skin test site.

Results of the test depend on your risk for TB. If you are at higher risk, a smaller bump is considered a sign of infection. People at lower risk for having TB need to have a larger bump to be diagnosed with a TB infection.

A positive reaction can usually be seen for about 1 week.

A positive tuberculin skin test doesn't mean you have contagious (active) TB. The test can't tell whether you have active or inactive (latent) TB. It also can't tell the difference between a TB infection and a TB vaccination (BCG vaccination). More tests—such as a chest X-ray, a sputum culture, or both—are usually done to see if you have an active TB infection.