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Tuberculin Skin Test

Results continued...

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.

A positive reaction usually remains visible for about 1 week.

Mantoux tuberculin skin test
Normal (negative results):

No firm bump forms at the test site, or a bump forms that is smaller than 5 mm (0.2 in.).

Abnormal (positive results):

A firm bump that is 5 mm (0.2 in.) in size suggests a TB infection in people who are in a high-risk group.

A firm bump that is 10 mm (0.4 in.) in size suggests a TB infection in people who are in a moderate-risk group.

A firm bump that is 15 mm (0.6 in.) in size suggests a TB infection in people who are in a low-risk group.

A positive tuberculin skin test does not mean you have a contagious (active) infection. The test cannot tell if the infection is active or inactive (latent TB). It also cannot 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.

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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