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Tuberculin Skin Test

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 TB antigen (purified protein derivative, or PPD) is put under the top layer of skin camera.gif. The fluid makes a little bump (wheal) under the skin. A circle may be drawn around the test area with a pen.

Do not cover the site with a bandage. You must see your doctor 2 to 3 days after the test to have the skin test checked.

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 have had tuberculosis (TB). An allergic reaction can cause a lot of swelling and pain at the site. A sore may be present.

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

After the test

Some redness at the skin test site is expected. The site may itch, but it is important that you do not scratch it, since this may cause redness or swelling that could make it hard to read the skin test. If itching is a problem, put a cold washcloth on the site and then dry it.

A strong positive reaction may cause mild pain. Talk to your doctor if you have:

Results

A tuberculin skin test is done to see if you have ever had tuberculosis (TB) (infection with Mycobacterium tuberculosis).

Redness alone at the skin test site usually means you have not 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 camera.gif (not the red area) is measured 2 to 3 days after the test to determine 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 in a high-risk group, a smaller bump is considered a sign of infection. People at low risk for having TB need to have a larger bump to be diagnosed with a TB infection.

Three levels of risk have been defined:

  • High-risk group includes people who have HIV, those who have had close recent contact with a person who has an active TB infection, and those who have symptoms or a chest X-ray that shows TB. Other people at high risk for tuberculosis include those who take medicines that contain corticosteroids for a long time or people taking tumor necrosis factor-alpha antagonists (used to treat rheumatoid arthritis and Crohn's disease).
  • Moderate-risk group includes people who have recently moved from or traveled in a country with a high rate of TB; those who use illegal drugs by injection (intravenous drug users); people who live in nursing homes; workers in hospitals, nursing homes, schools, and prisons; children younger than 4 years old; children (ages 4 to 18) who are exposed to high-risk adults; and homeless people. Others at moderate risk for having tuberculosis include people who are 10% or more below their ideal body weight and people who have kidney failure, diabetes, leukemia, cancer, or those who have had part of their stomach removed (gastrectomy).
  • Low-risk group includes people who do not have any possible exposure to TB listed in the other risk groups.

WebMD Medical Reference from Healthwise

Last Updated: April 04, 2013
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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