Tuberculin Skin Test
(TB Skin Testing)
- PPD (Purified Protein Derivative)
- Latent Tuberculosis Infection Test
- QFT (QuantiFERON-TB Gold Test)
A tuberculin skin test is used as a screen with people who may have been exposed to TB (tubercle bacillus), such as health-care workers, as a requirement for college entrance, IV drug users, or people who have recently immigrated to this country. PPD is also used to diagnose those people who may have a current TB infection, those who have been exposed and the infection has not yet become active (latent), and to identify those who are at risk of progression from latent to active state of infection.
A small amount of PPD (purified protein derivative) of the tubercle organism is injected just under the skin, and then observed for a reaction. A positive reaction indicates the presence of TB antibodies in the blood; a negative reaction indicates no antibodies in the blood and no prior exposure. However, a positive reaction will not occur until approximately six weeks following infection. While a PPD Skin Test is able to diagnose the presence of TB antibodies, there is no way to determine if the TB infection is active or latent.
A PPD Skin Test is also used to assess the status of the immune system; people with decreased immune status will have no reaction despite having TB infection. If positive results occur, then further testing is required. A PPD Skin Test should not be done if you are known to have active TB, since the reaction may be so severe that surgery may be needed to replace skin at the test site.
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