Tuberculin Skin Test
A tuberculin skin test is done to see if you have ever had Reference tuberculosis (TB) Opens New Window (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 (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 a picture of the Reference measurement of a tuberculin skin test result Opens New Window Reference Opens New Window.
Three levels of risk have been defined:
- High-risk group includes people who have Reference human immunodeficiency virus (HIV) Opens New Window, those who have had close recent contact with a person who has an active TB infection, and those who have symptoms or a chest Reference X-ray Opens New Window that shows TB. Other people at high risk for tuberculosis include those who take medicines that contain Reference corticosteroids Opens New Window for a long time or people taking tumor necrosis factor-alpha antagonists (used to treat Reference rheumatoid arthritis Opens New Window and Reference Crohn's disease Opens New Window).
- 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 (Reference intravenous Opens New Window 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 Reference kidney failure Opens New Window, Reference diabetes Opens New Window, Reference leukemia Opens New Window, 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.
|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 shows a TB infection in people who are in a high-risk group.
A firm bump that is 10 mm (0.4 in.) in size shows a TB infection in people who are in a moderate-risk group.
A firm bump that is 15 mm (0.6 in.) in size shows 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 can not tell the difference between a TB infection and a TB vaccination (Reference BCG vaccination Opens New Window). More tests—such as a chest X-ray, a sputum Reference culture Opens New Window, or both—are usually done to see if you have an active TB infection.
|By:||Reference Healthwise Staff||Last Revised: Reference April 15, 2011|
|Medical Review:||Reference E. Gregory Thompson, MD - Internal Medicine
Reference R. Steven Tharratt, MD, MPVM, FACP, FCCP - Pulmonology, Critical Care Medicine, Medical Toxicology