the test and in different batches of the same product may produce variable results (Villarino et al., 1999; Blumberg et al., 2000).
The possibility that repeated skin tests may stimulate a reaction in those with long-standing latent tuberculosis infection (a response called boosting). If people infected many years ago have not been tested recently, some will incorrectly test negative on a tuberculin skin test (a false-negative result). The test itself may then stimulate—boost—their sensitivity to tuberculin. If given a second test a few weeks or months later, these individuals may correctly test positive. This boosted reaction can be mistaken for evidence of exposure to tuberculosis and new infection. To control for this phenomenon, workplace surveillance programs may provide two-step baseline testing (two skin tests given 1 to 4 weeks apart). In this context, a positive result for the second test following a negative result for the first test is interpreted as a boosted result rather than as a true conversion. A worker with a boosted test result would not be included in a periodic retesting program.
The difficulty of interpreting test reactions in people who have been vaccinated against tuberculosis with BCG. Bacille Calmette and Guérin (BCG) vaccination is uncommon in the United States but fairly common in Europe and elsewhere.11 Thus, many immigrants to the United States have been vaccinated and may show false-positive reactions to the tuberculin skin test.
The geographic prevalence of other mycobacteria that produce reactions to tuberculin. In the southeastern United States (especially coastal areas) and other similar locales around the world, common exposure to mycobacteria other than M. tuberculosis can produce false-positive tuberculin skin test results.
The existence of anergy (failure of a person with tuberculosis infection to react to a tuberculin skin test). Approximately 10 to 25 percent of those with active untreated tuberculosis may have no reaction to the tuberculin skin test (CDC, 2000a). Anergy can be caused by immunosuppression related to HIV infection and certain other infections, poor nutrition, certain drugs and vaccinations, and a number of other factors. Although procedures that can be used to test for anergy exist, the interpretation and accuracy of these tests are uncertain, and CDC and others do not recommend their routine use (CDC, 1997; Slovis et al., 2000).
As described by CDC, “BCG vaccines are live vaccines derived from a strain of Mycobacterium bovis that was attenuated by Calmette and Guérin at the Pasteur Institute in Lille, France (29). BCG was first administered to humans in 1921. Many different BCG vaccines are available worldwide. Although all currently used vaccines were derived from the original M. bovis strain, they differ in their characteristics when grown in culture and in their ability to induce an immune response to tuberculin” (CDC, 1996c, p. 5). Although it is common in many European and other countries, the CDC does not recommend BCG vaccination except in rare circumstances.