Tuberculin Skin Tests: What To Think About

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What To Think About

• The results of a tuberculin skin test alone cannot confirm an active TB infection. Other tests, such as a chest X-ray and sputum culture, may be done to confirm an active TB infection when a skin test is positive. A person who has a positive skin test or chest X-ray, but no TB symptoms, is usually thought to have a TB infection that cannot be passed to others (latent TB). For more information, see the medical tests Chest X-ray and Sputum Culture.
• Among hospital workers or others who have periodic skin tests, a second test done within a few weeks of a negative test may be positive, even though the person was not infected between the two tests. These results (called the booster effect) may indicate a previous BCG vaccination or TB infection.
• About 5% of people who have inactive TB will develop active TB in 2 years. Another 5% of people with inactive TB will develop active TB in their lifetime. The chance of developing active TB is higher in children, older adults, and people with an impaired immune system.
• No more tests are needed for a person with a negative Mantoux test who has no symptoms of active infection and no history of being exposed to TB.
• Some people do not react to a tuberculin skin test even if they have tuberculosis. Conditions such as active TB, cancer, or autoimmune deficiency syndrome (AIDS) do not always respond normally to the TB antigens. In these cases, other skin tests may be done. If there is a skin reaction, then the tuberculin skin test is probably correct. However, if there is no reaction, the person's immune system is likely to be too weak to respond normally to the tuberculin skin test.
• A new blood test (called QuantiFERON-TB Gold) to diagnose TB has been approved by the U.S. Food and Drug Administration (FDA). This test may be used instead of a tuberculin skin test. For example, it can tell if a person reacted to a skin test because of an active TB infection or a previous BCG vaccination. However, this test is not yet widely done.
• A new rapid test (polymerase chain reaction, or PCR) for tuberculin testing of sputum has been approved by the U.S. Food and Drug Administration (FDA); however, more studies using this test are needed before it can be widely used.
• For more information about tuberculosis, see the topic Tuberculosis (TB).

Last updated:	May 18, 2007
Author:	Maria G. Essig, MS, ELS
Reviewed By:	E. Gregory Thompson, MD - Internal Medicine, Alfred A. Lardizabal, MD - Pulmonology and Critical Care Medicine/Tuberculosis
Editors:	Susan Van Houten, RN, BSN, MBA, Tracy Landauer