Tuberculin Skin Test
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 biologics (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.