circumstances has been deleted from the latest American Thoracic Society/Centers for Disease Control and Prevention recommendations.(19)

Although the sensitivity of tuberculin testing cannot be accurately determined in subjects without tuberculosis, a reasonable assumption is that the sensitivity is approximately that seen in patients with tuberculosis who have received adequate treatment and is approximately 95 percent. There are hypothetical reasons that the test might be more or less sensitive in people without disease. It is possible that people infected with M. tuberculosis without disease have better immunity to the infection and might be more likely to react to tuberculin. It is also possible that people who are infected without disease have a smaller antigenic burden and might be less reactive to tuberculin. The likelihood that either of these hypotheses is of major importance is small, and the sensitivity of the tuberculin test in latent tuberculosis infection is assumed to be approximately 95 percent.

Specificity

Just as with the sensitivity, the lack of an independent method of determining infection means that the specificity cannot be determined with complete accuracy. The major reasons for false-positive tests in uninfected persons are thought to be cross-reactions in persons who have been vaccinated with bacille Calmette-Guérin (BCG)(20) or persons who have environmental exposure to other mycobacteria.(21) Prior BCG vaccination is generally known, but environmental exposure to other mycobacteria varies widely geographically and is difficult to estimate.

The large scale skin-testing surveys in the past have shown a great deal of geographic variability in skin testing results.(11) In areas of the country where environmental mycobacteria are uncommon, the distribution of skin-test reactions approximates that shown in Figure B-1. This distribution is similar to results obtained in skin testing of patients with active tuberculosis, and the presumption is that there are few false-positive tests in such a population. At the other end of the spectrum, areas of the country with likely exposures to environmental mycobacteria more closely resemble the distributions seen in Figure B-2. In such a population there is no clear-cut unimodal distribution of positive results and many more false-positive tests are present. In such a population, false-positive results can be minimized and the specificity of the test can be improved by progressively increasing the cutoff point for determination of positivity. In the United States there is a tendency for results to resemble those shown in Figure B-2 in the eastern and southern parts of the country. However, considerable variability occurs even within a single state, and the U.S. population is much more diverse and mobile than it was when these results were obtained.



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