A TB skin test is also known as a PPD (Purified Protein Derivative),Mantoux, latent tuberculosis infection test or a tuberculin skin test. This particular test is used to determine if someone has developed an immune response to the bacteria that causes tuberculosis.
The tuberculin skin test is sensitive to both mycobacteria tuberculosis and mycobacteria bovis, both of which are known to cause tuberculosis in humans. The response to the skin tests will occur if someone has tuberculosis, if they were exposed in the past or if they received a BCG vaccine against tuberculosis.
The BCG vaccine is currently not given in the United States. It is, however, given in countries where there is a large population of people who suffer from tuberculosis. The vaccine is used to prevent the development of disease in young children. While it is effective in the youth it does not stop the development of tuberculosis in adults if they are exposed later in life.
The tuberculin skin test was developed based on the fact that the infection with Mycobacterium tuberculosis produces a delayed hypersensitivity skin reaction to certain components of the bacteria. The core elements of the tuberculosis skin test, also known as PPD, are parts of the organism placed into culture filter. Reaction to the PPD begins an individual when an immune cells called T cells, which have been sensitized by a prior infection, are sent to the sites where the skin test was performed.
There is a release of lymphokines, a chemical messenger, which cause a hard, raised area with clearly defined margins. This raised area contains fluid and other types of inflammatory cells.
These skin tests can show if you have a latent tuberculosis infection. This means that the infection is not active and that you are not contagious, but that you do carry the bacteria and it can become active at any time.
The skin test is administered by a health care provider who will use an extremely small and fine needle to inject some of the testing fluid just under the skin in the arm. It\’s important not to cover the spot with a bandage or tape and not to rub it or scratch it. If the spot begins to itch put a cold cloth on it.
After two to three days your health care provider will read this TB skin test. A reaction to the test will look like a raised bump. However a positive skin test requires several criteria which include defined borders of induration and not redness as well as a specific measurement of swelling. This means that not all TB skin tests which turned red or become swollen are positive.
A negative TB skin test means that you don\’t have tuberculosis and you weren\’t exposed. In some situations, your health care provider may request a second tuberculosis skin test at a later date to confirm the first results. A positive TB skin test means that you have the tuberculosis germs in the body. At this time, the physician will order a chest x-ray and other tests to see if the disease is latent or active.
All health care providers are required to have a tuberculosis skin test on an annual basis in order to protect themselves and their patients. Others should have a TB skin test if they have been in close contact with someone who has active disease, with in a country where many people have tuberculosis or have the HIV infection.
People who have had the BCG vaccine can still get latent or active tuberculosis. Although the vaccine can help protect young children the protection goes away as people get older.
RESOURCES
Center for Disese Control and Prevention: Tuberculin Skin Testing
Minnesota Department of Health: TB Skin Test
MedLine Plus: PPD Skin Test