The tuberculosis skin test is used to determine whether a person has been in contact with antigens from the bacteria that causes tuberculosis.
About the TB Skin Test
The tuberculosis skin test has several other names. It is also known as the tuberculin test, the purified protein derivative test for tuberculosis, and more simply as the PPD test.
This test is used as a diagnostic or more commonly as a screening measure to determine if someone has been exposed to Mycobacterium tuberculosis, the bacterium that causes TB. This test is used because someone who has been exposed to the bacteria will have a delayed hypersensitivity skin reaction in response to purified proteins from the species.
The test is conducted simply. A small amount of purified M. tuberculosis protein is injected into the top layers of skin on the forearm. The test is then “read" between 48 and 72 hours after the inoculation.
The Test Results
In someone who has been exposed to the bacteria previously, the immune system is primed with T cells that recognize M. tuberculosis proteins. When they are given the skin test, those T cells begin to gather at the site where the test was administered. They begin releasing chemokines, chemicals that cause the site to undergo a process called induration. The induration appears as a hard lump at the site of the test, with a raised red area surrounded by a paler area.
In short, if someone has been exposed to the tuberculosis bacteria, they will have induration at the site of a PPD skin test. Other signs, such as bruising or redness, are not counted as conclusive signs of tuberculosis exposure – the induration must occur for the test to be positive.
Test Interpretation Issues
Interpretation of test results can be problematic in some instances. What constitutes a positive result can change under certain circumstances. For example, the standard positive test in a healthy person is an induration that measures 15 mm or more in diameter. However, in someone with diabetes or kidney disease 10 mm or more is read as positive. People with compromised immune systems need only a 5 mm induration.
Another issue is that someone who has been exposed to the bacteria recently will often test negative. This is common in people who are newly diagnosed with TB.
Finally, there is the fact that people who have had the BCG vaccine for tuberculosis usually have a positive skin reaction to the PPD test. This is not an issue in the U.S., as the TB vaccine is not routinely given. In some countries, however, the TB vaccine is a routine part of the childhood vaccination process, and can mean it is difficult to interpret results of later TB tests.