Tuberculin Skin Tests: Results

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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 is a negative reaction meaning you have not been infected with TB bacteria. A firm red bump is a positive reaction to the skin test meaning 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 determine the result. Your doctor will consider your chance of having TB when looking at the skin test site. See an illustration of the measurement of a tuberculin skin test result .

Three levels of risk have been defined:

• High-risk group includes people with human immunodeficiency virus (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 with 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.

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 can not 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.

Last updated:	May 18, 2007
Author:	Maria G. Essig, MS, ELS
Reviewed By:	E. Gregory Thompson, MD - Internal Medicine, Alfred A. Lardizabal, MD - Pulmonology and Critical Care Medicine/Tuberculosis
Editors:	Susan Van Houten, RN, BSN, MBA, Tracy Landauer