The results of a tuberculin skin test alone cannot confirm an active TB infection. Other tests, such as a chest X-ray, sputum cytology, 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).
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 TB infection that occurred a long time ago or a previous BCG vaccination.
About 5% to 10% of people (1 to 2 people out of 20) who have inactive (latent) TB will develop active TB at some time in their lives.1 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 tuberculin skin 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 acquired immunodeficiency 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. But if there is no reaction, the person's immune system is likely to be too weak to respond normally to the tuberculin skin test.
Rapid blood tests to diagnose TB have been approved by the U.S. Food and Drug Administration (FDA). These tests may be used instead of a tuberculin skin test. A rapid blood test may be able to tell if a person reacted to a skin test because of an active TB infection or a previous BCG vaccination. Rapid blood tests are also called interferon-gamma release assays (IGRAs).
Rapid sputum tests that can detect TB bacteria in sputum have been approved by the FDA. These tests, called nucleic acid amplification tests (NAAs) can provide results within 24 hours. But they are done only when a person is strongly suspected of having TB.