What the risks are:

There is a very small risk of severe redness and swelling of the arm in people who have had a previous positive PPD test and who have the test again. There also have been a few cases of this reaction in people who have not been tested before.

Special considerations:

A positive skin test does not necessarily mean that a person has active TB. More tests must be done to check whether there is active disease.

Many people who were born outside the United States may have had a vaccine called "BCG," which can lead to a false-positive test result. However, most experts say that a past BCG vaccination should not change the PPD result when the test is done in people with an increased risk of TB infection or disease.